Don't swim near any pipes, weirs, dams, water board equipment etc - the machinery may be under remote control. The water will be cold, and you will get tired and get cramp very quickly. If you swim from a boat make sure you can get back in when you are tired. Read local information. Obey the lifeguards and flags on beaches.
Some beaches have separate areas for swimming and other water sports.
Be particularly aware of Weils disease and sewage outlets. Try not to pollute the water yourselves. This includes using soap or detergent in high-altitude tarns.