Mostly fundraising for good causes, many even do it in fancy dress.
But one determined adventurer donned a wetsuit, battled freezing conditions and was constantly stung to become the first ever to make the epic journey by sea, completing it at 12.06pm yesterday (Mon).
Iron man Sean Conway, 32, covered almost 1,000 miles in 136 days, swimming up the west coast of the country, taking an estimated three million strokes and losing a stone in weight in the process.
Despite previous extreme adventures which have seen him cycling 16,000 miles and climbing Africa’s highest mountain, Kilimanjaro – bizarrely dressed as a penguin – he admitted the swim was the most difficult daredevil project he had ever attempted.
This is, in part, because he suffers from seasickness.
Moments after emerging from the freezing sea at John O’ Groats, he said: “It just shows that if you put your mind to something anything is possible.
“The hardest part was trying to deal with the weather, the cold and jellyfish in the face. I had to grow this ridiculous beard to stop the stings.”
The incredible achievement will, however, not be recognised by the Guinness Book of Records or open swimming bodies, because he wore the wet suit and, on occasion, flippers.
He spent a total of 90 days swimming, with the remainder being forced to stay on dry land because of the bad weather.
Apart from the seasickness, other hazards included jellyfish - he was stung at least 10 times - and swallowing up to 50 litres of sea water.
He was meant to be on a strict diet to boost his calorie intake, but his jaw has recently been so cold he was unable to chew solid foods and has had to have meals liquidised.
His achievement has also taken two months longer than expected, with the bad weather, tiredness and shorter hours of daylight all playing their part.
But it was all worth the effort. He broke down in tears after walking out of the water at John O’Groats harbour, to be sprayed with champagne by around 50 delighted supporters.
Mr Conway, who was born in Zimbabwe but now lives in the South of England, said he undertook the challenge because people doubted it could be done. He has raised thousands of pounds for the War Child charity in the process.
He swam around 10 miles (16km) a day and slept on a yacht or in accommodation on the shore.
He said: “No-one’s ever done it and that sort of got me thinking, considering Land’s End to John O’Groats is such an iconic route. Then, as soon as people told me I was going to die or that is wasn’t possible, I just thought ‘I’m going to prove you wrong’.
“The weather’s been quite tough - big winds and big waves have slowed my progress a bit.
“Getting stung by jellyfish is quite tough. I’ve had to grow a ridiculous beard to shield my face and had to cover up with loads of Vaseline because every now and again I’d take a stroke and get a jellyfish right in the face and I don’t know who gets more of a fright, me or it.
“We’ve lost kayaks at sea, rigs at sea and our boat has taken a bit of a hammering. We all took a bit of a hammering. The sea can be a nasty place.”
The 32-year-old from Leckhampton, Gloucestershire, had not swum more than three miles when he set off from Land’s End on 30 June - admitting that he suffered from sea sickness.
But he finished the final 500 yards, battling 30 mph winds and a moderate sea swell.
He added: “I am not bothered about the Guinness Book of Records or the open swimming lot. There have been a lot of negative comments because of the wet suit and fins. But I could not have done it without a wet suit.
“I did not do it for the record books. I did it for different reasons - for me and to prove it can be done and that anybody can do anything if they put their mind to it. I have proved a lot of doubters wrong - though I’m not sorry it’s over.”
Mr Conway’s 60-year-old mother Babette, who was also at the finish, said: “I’m so proud of him. This is his biggest adventure - he has a strong will and a strong mind. At one time I think I was the only person - other than Sean - who thought he could do it. But I knew he could.”
During the marathon swim, he has received good luck messages from comedian and fellow long distance swimmer David Walliams, funnyman and charity raiser John Bishop and entertainer Denise Van Outern.
He said: “I’ve been dreaming of swimming into the harbour at John O’Groats for some time now - it was on my screen saver.
“I was supposed to be eating 5000-6000 calories-a-day but I got eating fatigue which meant all my meals had to be liquidised.”
Each time Mr Conway left the water, his co-ordinates were mapped and the support boat returned to the same spot to resume the swim when it was next safe to do so. But that drop-off point could be sometimes two or three hours away from where Mr Conway and his team had to take shelter on land, adding to his frustration over loss of time.
But he said: “The highlights have been how beautiful the British coast is and how friendly the people who live in the coastal communities are. I have been overwhelmed by their kindness.”
Mr Conway has raised over £6000 from the swim so far for War Child, a charity that helps children affected by war.
He has dismissed suggestions of swimming down the east coast of the country to return home, adding: ”I’ve had enough of swimming for a while.”