In my state if you swim in any body of water it's almost always tainted somehow, maybe not with sewage but perhaps some other toxic pollutant. The only exception I can think of is the man-made reservoirs used for drinking water that people swim...
even in a natural body of water, they have parasites in them, like amoebas, gardia, parasitic eggs,,,etc. because animals die and shit in the water too. and parasites love using aquatic hosts like snails and crustaceans, and amphibians.