With its clean lochs, tarns, rivers and sea, Scotland is experiencing a renaissance in wild swimming

Paying such close attention to the scenery is a strategic attempt to ignore what's happening to my body, submerged in the icy Firth of Forth. My Adam's apple bobs at the surface and a stinging salt wave slops into my mouth and eyes. I'm trying to breast-stroke in the direction of the North Sea but the tide keeps pushing me back towards the beach and my arms and legs aren't coping well with the temperature.

I'm not just cold, I'm in pain – a throbbing, squeezing soreness which Amy Clark, 29, swimming in a bikini to my left, likens very accurately to "an ice-cream headache all over". Yet according to her 28-year-old flatmate Jen Trendall, also in the water, this is much warmer than some of the freshwater swims they have enjoyed. "I remember when we went in Loch Etive," she says, "I felt like my heart was about to stop."

The mutt-walkers and pre-tea strollers on the beach gaze out at us incredulously, as well they might. So-called "wild swimming" in Scotland, even at the start of summer, is by no means straightforwardly pleasant; its pleasures, like our coastline, are complex and sometimes hard to fathom. Yet for growing numbers of Scots, swimming in lochs, rivers, firths, seas and sounds is part of their everyday happiness, a deep engagement with home. "There is a beautiful sense of calm about swimming in the wild," says Trendall. "It's good for your heart, soul and skin."

It is difficult to say just how many people in Scotland are into wild swimming. The trend in a Scottish National Heritage annual survey of how people are using the countryside is that only one per cent of respondents swim in the sea, rivers or lochs. But by its very nature, wild swimming is unregulated, unobserved and often unthinking. You might dive into a cooling plunge pool on the way back down from bagging a Munro and wallow happily for half an hour. That's a much less self-conscious act than someone deciding to swim across the whirlpool at Corryvreckan or into the sea caves of Brei Holm in Shetland, but it is still wild swimming.

This has, of course, been going on for a long time. Before there were public indoor pools, swimming outside was simply what you did. Growing up in Aberdeenshire, Lord Byron swam in the Don and Dee. The great Scottish mountaineer WH Murray had fought in the Libyan desert and been a prisoner of war in Italy and so knew all about the pleasures and consolations of cold water. During the summer heatwave of 1949 he crossed Rannoch Moor naked, bathing in Loch Laidon as he went; on a sweaty June expedition in the Cuillin of Skye he launched himself down the scree-shod foot of An Stac, stripped and plunged into Loch Coire Lagan. "I felt that I had never awakened till now to that allegedly familiar quality of water – its wetness," he later wrote. "That there was beauty in wetness had hitherto never penetrated my mind."

During a recent trip to Angus I met a taxi driver in his late sixties with sour eyes whose face and voice freshened when the subject turned to the swims of his childhood. "We didn't have a pool in Brechin," he said, "but we had the River Esk." He remembered swimming along to a deep point where he and his pals would dive back in from the steep bank. "Once you had mastered that you graduated to the Brechin Bridge."

He climbed up, pushing his toes into holes and cracks in the stone, and then leapt off the parapet into the water. It was frightening, but once you had started climbing, it was the only way of getting back down with any chance of safety. "Aye," said the driver, shaking his head as we passed the entrance to Glamis Castle, "those were some of the best days of my life and I didn't realise it at the time."

Public baths began to be built in Scotland in the 1870s, many of them exclusive members-clubs; in those that weren't, the emphasis was on bathing rather than swimming. It wasn't until the 1950s that councils began building heated indoor swimming pools in large numbers in a drive to improve public health. At one time, every city and most sizeable towns would have at least one pool. This period of growing prosperity was the beginning of the end for swimming outside.

"We've softened up as a population because we've got hot pools and central heating and because we go on foreign holidays," says Kate Rew, 39, founder of the Outdoor Swimming Society. "As a result, natural temperature water has become a shock to some people. Fears about pollution also put people off, though in fact the rivers now are doing really well in terms of cleanliness. And health and safety is a really big thing. Landowners are afraid of being sued and that has led them to put up 'Danger: No Swimming' signs everywhere." Legally, Scottish wild swimmers have an advantage over their English counterparts as the Land Reform (Scotland) Act of 2003 established a right of responsible public access to all inland water for recreational purposes.

Rew runs outdoorswimmingsociety.com, one of two popular websites dedicated to alfresco dips, the other being wildswimming.co.uk, both of which also have busy groups on Facebook. There are 4,275 members of the Outdoor Swimming Society and Rew expects this number to grow to 10,000 over the next few months. "This summer everyone's going to be holidaying in Britain, it's supposed to be hot, and we're all feeling poor," she says. "So this is wild swimming's time. It's free, it makes you happy, and it's right on your doorstep. What more do you want in the middle of a recession?"

She believes, though, that the rising public interest in wild swimming is down to more than economic necessity. "We're very constrained in our daily lives and jobs. Society places a lot of demands on the individual to be a civilised person, but ultimately we are just animals and everybody, once in a while, wants to experience the physical, non-thinking part of themself. Going wild swimming allows us to reset ourselves on some kind of base level."

It's also arguable that outdoor swimming's burgeoning popularity is being boosted by a general anxiety about the environment which has seen a rise in public interest in all things wild, from mountaineering to mushroom foraging. As we feel the world is slipping from our grasp, we are trying to cling closer.

But everyone has their different reasons for taking the plunge. For Steph Mackie, a student in Glasgow, the epiphany came on holiday last year while swimming in Lake Bled, high in the Julian Alps. Now she thinks nothing of swimming in Mugdock Loch first thing in the morning and Loch Lomond in the evening.

Joe McNally, a novelist and business consultant in Motherwell, is planning to make his maiden dip some time soon. In the past he has had some problems with depression and swims indoors in part to help his mental health. He believes that swimming in wild water would be even more therapeutic, partly because of the beautiful surroundings and partly because of the element of risk.

The renaissance in wild swimming seems also to have been caused by Waterlog, a book in which the late writer and environmentalist Roger Deakin detailed in lyrical prose his attempt to swim through Britain. It was published in 1999, a pebble thrown into a millpond, and in the decade since its ripples of influence have continued to widen. "I've never known a book that has had such an effect in terms of changing the behaviour of people," says Deakin's friend, the writer Robert Macfarlane. "Thousands of people have thrown themselves into water as a result of reading it." Macfarlane remembers swimming in a remote loch in Sutherland and meeting, by complete coincidence, a woman who had just been listening to an audio-tape of Waterlog.

Deakin died in 2006 and since his death his former home in Suffolk, an Elizabethan farmhouse surrounded by a moat, has become a place of pilgrimage for wild swimmers who regard him as the godfather of their obsession. "I swam with him lots of times in the moat," says Macfarlane. "The last time I swam there was just before his diagnosis with a brain tumour and he didn't want to be in the water. But the year after he died we had a memorial day and the moat was just full of children and adults swimming and splashing."

Among its other influence, the success of Waterlog has led to a flood of practical guides being published on wild swimming – attractive volumes, perhaps more suited to the coffee table than the rucksack, full of photographs in which Scotland could easily be mistaken for a Greek Island or the set of a condom ad; buxom gals stroll across beaches in Sutherland and attractive young couples wade waist-deep in water which, being some miles north of Ullapool, is more likely to induce detumescence than desire.

In fact, scientists now believe that cold water actually increases sexual appetite and fertility. But standing in the Forth at Gullane, I felt lividinous not libidinous. If bed came to mind, it was as a vision of a heavy duvet and hot-water bottle. As Kate Rew laughingly puts it, the usual Scottish temperature range is "cold, bastard cold and freezing".

Some wild swimmers grow addicted to the endorphin rush associated with an icy plunge. Robert Macfarlane used to be a "cold water fetishist" then, one December, he swam in a frozen lake in Beijing. The air temperature was minus five celsius and the water, once some of the inches-thick ice had been cleared with boiling water and axes, was so cold it burned. "I lost my sight temporarily but almost totally within 10 minutes of getting out," he recalls. "The spectrum of colours narrowed to green and white only and the field of vision narrowed as though I was looking through a little round keyhole. A coroner later told me that the cold water makes the arteries that run up the back of the neck to supply the brain and the optic nerves constrict, so you get a time-delayed blackout."

For Rew, fear of cold water is something that wild swimmers simply must conquer. "We need to rebrand cold a bit," she says. "It is a bummer for a lot of us that the water's not warmer but it's even more of a bummer to have to wait till we go on holiday to have a swim. So I think you've just got to suck it up a bit. It's psychological. Instead of thinking, 'Yikes, it's cold!' think 'It's enlivening, it's refreshing, it's invigorating.'

"I still have Eeyore moments when I'm standing on the shore thinking 'I'm not sure I want to go in'. But the thing to do is get in and stay in for 90 seconds and after that your circulation will kick in. Instead of focusing on the cold, try to have a purpose like swimming to a rock. The other great tip is to exhale as you get in.

Particularly in Scotland, the water will take your breath away. But if you blow out first you'll find that you can breathe. I went through a period when I didn't swim because I felt I had something physiologically wrong with me that meant I couldn't cope with really cold water. But it was just my chest contracting and I was panicking."

According to Daniel Start, writer of two influential guides to wild swimming, the sea is at its warmest in September. He says that though it is possible to swim all year round, the season proper runs from Easter to Halloween. "You can stay in for 20 minutes or so without any problems," he says. "You should come out if you start shivering.

There's a short-term health benefit to cold water, though, because it

squeezes all the blood out of the muscles and gives you a whole replenishment of blood, which is why athletes take a cold shower before a run." Through regular swimming, he says, you can teach your body to get used to the cold. "And cold adaptation is very good for the cardiovascular system."

Despite the temperature, Scotland is reckoned by aficionados to offer the best wild swimming in Britain. "It's a Paradise," says Start, who is 36. "Every loch, tarn and beach is so much more beautiful and clean than you find in other parts of the country." It's also excellent for skinny dipping, he reckons, because there are more isolated areas with very low populations. "And the Scots are much less prudish than the English about that sort of thing."

He cites as his greatest ever swim, the iconic Fingal's Cave on the island of Staffa. "I was diving into this beautiful clear water, and there were people singing so there was this resonance coming through as well. Each of the basalt columns acts as a step so you can choose the height that you want to dive in from."

In his writing about the best places for wild swimming, Start is always keen to emphasise those lochs and natural pools where historical and literary figures swam and bathed. Swimming in the footsteps, so to speak, of famous people seems a strong motivation for many wild swimmers. It's as though immersing oneself in the landscape is also to get a sense of plunging back through time, which makes sense as waterways change much more slowly than other parts of the country.

Charles Sprawson, in his classic history of swimming Haunts Of The Black Masseur, relates how he travelled around America and Europe swimming where Tennessee Williams had swum; Williams himself was buried at sea in the Bay of Mexico as close as possible to the spot where the poet Hart Crane had drowned. Byron swam the Hellespont, the strait dividing Europe and Asia, because in Greek myth it was the body of water which Leander crossed in order to woo the priestess Hero. Now people swim it because Byron did.

In Scotland, the Fallach Falls near Inverarnan, are a popular spot because of the legend that Rob Roy bathed there in order to rid himself of hangovers.

This sense of tradition is particularly acute within the few remaining outdoor swimming clubs in Britain. The last club in Scotland is Ye Amphibious Ancients Bathing Association, members of which – known as the Phibbies – have been throwing themselves into the Tay from Broughty Ferry pier since 1884. Walking from the town centre down to their club house at the water's edge, I pass the Broughty Ferry Baptist Church which was built only three years before the swimming club was founded, and it's hard not to regard what the Phibbies do as a kind of secular baptism. At one time it was the done thing to start each day, excepting the Sabbath, with a swim in the river. Song lyrics framed in the clubhouse celebrate this daily ritual. 'There's a club known as the Ancients an' they dook at Bro'ty Pier/Its members are good men and true.

They're famous far and near/They leave their beds so early, an' nae storm nor cauld they fear/When they gaither for the dookin' in the mornin'.'

The Phibbies have around 90 members ranging in age from primary ones to pensioners. Unlike the wild swimmers who wallow in lochs and bays, the club has a competitive edge. Among many annual swim races there is the Tayport Cup, awarded to the fastest person to swim a mile across the river from Tayport back to Broughty Ferry, and the Double Tay, a race to Tayport and back. The male freestyle records for both were set in 1957 by, respectively, Paddy Hayes, an army man from Arbroath, and Bob Sreenen who had competed in the Olympics at Helsinki and Rome.

There are people at the club who remember that day, among them Netta Spence, the Honorary Chief of the Phibbies, who is 76, and has been a member since 1950. The following year, she was among the first group of swimmers to cross the Tay since the outbreak of the Second World War. "We chattered the whole way across," she says. "We decided that if we weren't finished by half-past eight we would come out and into the boat because that's when the second house of the Regal started." In 1952 she swam the Double Tay and remembers proudly that she beat all the men.

"Are you going to take a dip?" Spence asks me.

Well, yes, but the trouble is that the harbour at Broughty Ferry is not like the beach at Gullane. For one thing there's a dead bird floating next to a crisp poke amid some scummy froth. There are six Phibbies in the water, bathing caps bobbing as they train for the Tay crossing. The water is accessed via a steel ladder attached to the harbour wall, but once I'm waist deep, bladderwrack floating round me like a grass skirt, I realise that not only can I not touch the bottom, I can't see it, and the only thing I can hear over the wind is the echoing memory of advice Joyce McIntosh, Chief Ancient, gave me as I walked out of the clubhouse: "You must stay in the harbour. If you go in the river you're gone and you'll end up in Arbroath." Worse even than the threat of Arbroath is the water itself which is intensely cold. Even my substantial layer of bioprene – wild swimmer slang for body fat – is no comfort. Anxiety floods over me and, blushing, I climb out.

My borrowed bathing yellow cap suggests cowardice, which feels appropriate, but I just didn't feel safe in the water. It would be irresponsible to deny that swimming outdoors is a riskier business than swimming in a pool. However, there are things you can do to make the experience safer; a selection of tips are listed at wildswimming.co.uk.

Standing in the doorway of the clubhouse, I greet the Phibbies as they come out of the water. Invariably, they are shivering and they all tell me how great they feel. They include in their number Andrew Leslie, 12, who last year swam across the Tay for the first time, and Steve James, 47, who is in training for his first crossing; his partner's father died suddenly two months ago and so he has decided to seize the day and complete a schoolboy ambition. He's feeling the cold, though. Last time he swam he was shaking so violently afterwards that he couldn't hold a cup or the steering wheel to drive home. "I love it," he says.

Jessie Kidd, 49, is dripping and buzzing. "That was brill!" she exclaims, throwing her arms out towards the water from which she has just emerged. "See what I mean about the colours? You're in another world." Earlier, she had eulogised the beauty and delightfully altered perspective you enjoy from the Tay. She works in the social work department, a stressful job, and the swim helps to clear her head. "It's total therapy," she says. "The water's in charge and you've got to respect that. It's your opponent but also your master. You're accepted, but just accepted. It allows you to be part of it, and that's thrilling."

The sun is setting and the tide beginning to go out as I leave Broughty Ferry. The streetlights are coming on, the castle forming a dark silhouette, and the Phibbies, like all Scotland's wild swimmers, are beginning to look forward to another day in water that feels alive on their skin and makes them feel more alive for being in it.

Wild Swim by Kate Rew is published by Guardian Books, priced 12.99. Wild Swimming: Coast by Daniel Start is published by Punk, priced 14.95

GREAT WILD SWIMS IN SCOTLAND

Loch Coruisk

A remote freshwater loch on Skye, the Cuillin reflected in its peaty water, which is accessible only by boat from Elgol or by a challenging seven-mile walk. One of Scotland's most atmospheric swims. Skye is also home to the famous Faerie Pools – incredibly cold, clear and magical. Beware the midges, though.

Seacliff

This quiet East Lothian beach has views of the ruined Tantallon castle and out to the Bass Rock. You can also swim through the inlets of the tiny and remarkable harbour here which was blasted out of the red sandstone cliffs in the 19th century.

The Gulf of Corryvreckan

The water dividing Jura from Scarba is notorious for its powerful whirlpool in which George Orwell came close to losing his life. The adventure holiday company Swimtrek offer accompanied swimming tours.

Falls of Falloch

Also known as Rob Roy's Bathtub, this waterfall near Crianlarich is only a short walk from the A82. At the foot of the waterfall is a huge plunge pool. It's a popular beauty spot and part of the West Highand Way so don't go expecting a quiet commune with nature.

Sandwood Bay

In the far north near Cape Wrath and accessible by a four-mile walk, this is about as remote as it gets so there's a good chance of having the mile-long beach, arguably Scotland's best, to yourself. Out to sea there's the huge and dramatic sandstone stack Am Buachaille.