In the Valais there is a small lake called La Gouille du Duzillet close to St Triphon. The lake is small, easily swam from one side to the other. It has a maximum depth of around 17 meters. 

Along the side of the lake you have quite a bit of aquatic life. With a friend we went there and spent over an hour and a half going from one school of fish to another. Some were quite large whilst others were tiny. Occasionaly you'd see large schools with predatory larger fish swimming close by. 

It may be because I watched so many attenborough documentaries but I think we may have witnessed the larger fish feeding on the smaller ones if we'd waited a little longer. It's an easy, cheap sport to practice and you don't need to be such a good swimmer. Do with this information what you like. 

Sorry for sidetracking you into this train of thought ;-)