The swimmers included two-time world champion Amber Keegan, Olympians Toby Robinson and Hector Pardoe, and ultramarathon swimmers Jessika Robson and Daniel Smyth.
Ms Keegan said: “This was one of the toughest challenges I’ve ever taken on, but what shocked me most wasn’t the distance or the fatigue, it was the state of the water.”
She said some points along the river were “absolutely disgusting”.
“Nobody should have to risk their health just to enjoy their local river. It’s 2025, and this simply isn’t acceptable.”
Mr Maclean called it a “tough week that tested us all”.
He added: “The fact we literally couldn’t swim the full route proves our point, and I hope one day an uninterrupted Thames relay will be possible.”