One way round this for a swimming animal is to develop an outlet which allows urea to dissolve directly in the water as itswims, which is what the turtles seem to have done.
She says she chose Lake Ontario for her swim because it's one of the hardest open-water swims in the world, and she wanted to "accomplish something difficult" for children who fight cancer.
Although training in a pool is an essential part of an open-water swimmer's regimen, it doesn't prepare one for the kinks of swims longer than 10 miles, or at temperatures below 60 degrees.