It was fitting that Glasgow-born Grainger, Britain's most successful female oarswoman, should win the crucial gold as she made it five world titles in a distinguished career. Marcus Bateman and Matt Wells, in the men's double sculls and the men's eight, had to settle for silver when both could have won gold while the women's eight were disappointed only to finish fourth.
Grainger's first world title came in the women's pair in 2003 and she was part of the quadruple sculls crew that won gold for three years in a row between 2005 and 2007.
The 34-year-old, who took up rowing while at Edinburgh University, said: "Every one is very special and very individual. I'm hugely proud of this one, this is the most successful season I've ever had, so it just keeps getting better."
Grainger and Watkins first tried out the double in January and it has been an instant success, with three World Cup wins earlier this season positioning them as the clear favourites for gold. And they never looked like faltering as they took an early lead and then extended it all the way to the finish lane to leave Australia and Poland to battle it out for silver and bronze.
Grainger continued: "It's always a bit of an honour to come in as favourites, it shows we've had a very successful season so far. We always felt there were going to be some surprises within the field so we definitely didn't come into it complacent. We really got ourselves up for the best race we could put out there."
Watkins was thrilled to be able to enjoy the moment of securing her first senior world title rather than having to scrap for the line. The 27-year-old said: "We knew we had a strong start but we hadn't shown what we could do through the middle. We got into our rhythm and we just felt in control."
The men's eight put in a tremendous performance in the final race of the regatta and were closing in on defending champions Germany with every stroke as the line approached. However, it was not quite enough, with the Germans winning by less than a second while Australia took bronze.
The result also meant 38-year-old Greg Searle, who was back competing at his first World Championships following a decade in retirement, was unable to clinch a second world title 17 years after his first. Searle said: "We raced really well. I'm very proud of what we've done as a team together. It's a step in the right direction, it's not gold yet, but step by step we're moving up the field.
"We've been on the podium every time this year and I know how special that is.It's a very special thing that for most people only comes around once in a lifetime and it's lovely to get a second bite."
In many ways Bateman and Wells were even closer to winning a first gold as they entered the final stretch with a half-length lead over New Zealand pair Nathan Cohen and Joseph Sullivan. But the home crew found an extra gear when it mattered most to surge past their rivals, and Wells admitted the noisy crowd had played their part.
He said: "We thought we had it all the way down for 75/80 per cent of that race. Unfortunately, when we were coming in and going through our big gears, the Kiwis just had a little bit more than us and maybe that's down to their home crowd. Although you do block it out of your head when you're racing in it, I think they got to us."
That made it 11 medals for Britain, more than any other country and easily their best World Championship tally, with nine coming in Olympic classes and two in Paralympic. The only real disappointment on the final day was the failure of the women's eight to get a medal as they found Romania just too strong in the race for bronze behind the United States and Canada. A tearful Olivia Whitlam said: "We weren't good enough and that's just life. You go away, back to the drawing board."