do you think swimming in water that fish live in is disgusting ?

Not really...it won't hurt you and people have been doing it for thousands of years so. 

no, not at all.

That's the same water that you drink, after lots of people and other animals have also "gone to the bathroom" in it. Bon apetit.

Lakes, rivers and oceans contain a whole lot of water.

No, you want living things because they are like the "canary" which shows that the chemicals (which people are polluting into the water) aren't too bad. Or, as the Yankees say, "the solution to pollution is dilution". That's their solution. 

I swim in the sea whenever I can. It is much cleaner than pools that humans pee in.