Open water swimming looks truly gorgeous from the shore. On television it is even better: the sun glittering off the apparently azure waters off Copacabana beach with Sugarloaf mountain and the rest of Rio’s spectacular sponge-finger skyline behind. Short of Pelé arriving on a pedalo, every other recognisable Brazilian attraction was sparklingly present and correct.

In reality, of course, it was a cunning illusion. Out there, beneath the beautiful veneer, is a sport that – regardless of the water quality – frequently turns dirty. There always seems to be at least one contentious incident at every Olympics and, sure enough, another ugly episode duly unfolded in the final stretch of the 10km slog around the world’s most scenic course.

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Nothing was ever going to stop the impressive Sharon van Rouwendaal from winning a convincing gold in 1hr 56:32.1min but, 16 seconds behind the 22-year-old Dutchwoman, France’s Aurélie Muller and Italy’s Rachele Bruni found themselves in one of those death‑or-glory rushes to the line familiar to anyone who has ever challenged one of their immediate family to a winner-takes-all race in the holiday pool. Muller, in her desperation, dunked the Italian to stop her touching the board first and was summarily disqualified, prompting Brazil’s fourth‑placed Poliana Okimoto to be promoted to bronze, becoming her country’s first female swimming medallist.

Bruni felt justice had been done – “She pushed down my arm at the finish … she obstructed me and I could not touch the board” – but Britain’s Keri‑Anne Payne, who was bumped up to seventh place, summed the situation up well afterwards: “To be disqualified in the last bit of the race, especially when you’ve won a medal, is heartbreaking. I can’t even imagine how the French girl is feeling.”

Payne also admitted to a range of emotions, having finished outside the medals for the second successive Games after claiming silver in Beijing in 2008. While she was proud to have swum a competitive race in what is likely to be her final Games, she could also have done without the flawless weather and calm seas that greeted the 26 competitors as they assembled apprehensively on the beach beforehand. “I was hoping for rain and cold,” she said resignedly.

“If the course was windy and wavy it might have been a completely different result. Two days ago the start pontoon, this massive concrete thing, washed up on shore. I was getting really excited. I was like: ‘Yes, the winds are coming.’ Then today it was really flat, which isn’t my conditions. It totally played into the pool swimmers’ arms. I was racing a world championship silver medallist in the 400m freestyle.”

It was not an entirely straightforward procession, even so. The girl from Ipanema was certainly never required to pull on her goggles and do four 2.5km laps of what was theoretically the most polluted Olympic course in history. Never mind the green water at the diving pool, this was meant to be seriously dicey; surely it was only a matter of time before Payne bumped into a submerged sofa or something rather more decomposed. Given visitors to Rio have been advised not to put their head under the water for fear of catching one of the umpteen viruses floating around, this felt like the type of event for which every competitor should have had a medal simply for entering.

In the event, like so much surrounding marathon swimming, the truth was slightly more prosaic. Payne reckoned the water was “totally fine” and said she had swum in much worse, not least off Jinshan City Beach in China a few years back, when there were dead dogs in the water. She did concede, though, it had been an unusually niggly race –

“I’ve never seen so many yellow cards as we went through the race” – and admitted she had missed a trick at the end of the third lap. “Going around the buoys was absolute carnage. I was looking around and thinking: ‘Where am I? What am I doing?’ Then I looked up and they were gone. It only takes a split second and that’s exactly what happened … I could see the Dutch girl in front … to know I couldn’t really catch her up can be heartbreaking but for me it was a good spur. To catch up the girls I did towards the end was a great achievement.”

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The personable Payne, now 28, has certainly been a fine ambassador for her sport and, in addition to various business projects including helping people prepare for sporting challenges of their own, she is keen to encourage more people to take up open water swimming. “In the UK alone there are at least half a million people who are taking up open water swimming – from the Channel, to all the great swims happening around the country,” she said. “It is incredible to think that eight years ago nobody knew what it was. They thought, ‘You what? You’re swimming outdoors?’”

Even she, though, could not quite believe this particular backdrop. “The most amazing thing for me was swimming along the back of the course, looking to my left and seeing Corcovado with Christ’s arms outstretched. It was amazingly cool.” It is hard to recall anyone waxing quite so lyrical as they clambered out of the murky Serpentine four years ago.