Endurance swimmer Lewis Pugh has slammed the government for allowing water companies to pump raw sewage into rivers and seas.
Lewis, 51, who took on the toughest swim of his life in September to cross Greenland's Ilulissat Icefjord, said "it's criminal".
"We have a massive problem. We're using them as sewers. It is ecocide," he said.
"It's utterly filthy. I've done a few river swims in my life and every single major river swim I've done, I've gotten sick - by rivers not only here in the United Kingdom but worldwide.
"We are damaging the most precious thing that we have, which is water.
"When you see the way people treat our waters, treat our rivers...it's criminal."
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He added: "I think the government got this wrong. The government underestimated the feeling of the public on this.
"When you allow private water companies to pour raw sewage down our rivers and into the sea, they won't put up with that kind of nonsense."
He was speaking at thinkBeyond’s day of Sport & Climate Action at the Extreme Hangout, one of the fringe venues at COP26.
Lewis, who grew up in Plymouth but now lives in South Africa, says he would welcome legislation that would enable polluters to be sued.
"Rivers are the arteries of the world," he said. "I'd love to see all these rivers be given legal status and jurisdiction to be able to sue those companies which are actually now polluting and damaging them so badly."
In 2020, Lewis became the first person to swim under the Antarctic ice sheet, swimming in icy water just above 0C with an air temperature of -25C for eight minutes.
He says even re-watching one of his ice swims makes him feel physically cold from the memory of it.
"When you've been really, really cold you never quite warm up. It's deep in your bones, you remember it. Even watching that video, I was cold down to the inside of my bones. "This sport is the only sport that the more experience you have the harder it becomes."
He has witnessed dramatic changes in the poles over the years.
"It’s really fast moving. I remember the first big swim I did in the Arctic was in the Norwegian Arctic. When I first swam there the water was 3C. I went back there recently, it's now 10C. It’s gone from 3 -10 in just 12 years.
"I’ve also just come back from Greenland where I undertook a swim across the face of the fastest moving glacier in the world. That glacier is now moving at a speed of 40 metres per day."
He added: "The glaciers are moving faster than the politicians."
Environment Secretary George Eustice said a new legal duty would be imposed on water companies to "progressively reduce" the amount of sewage pumped into waterways.
