Warm and cold are relative. Anything above 18 degrees is definately swimmable and if the weather is really hot (over 25) people crawl into even colder rivers, lakes and sea to cool themselves off. Even the lakes are usually shallow for the first 50 meters, making the ideal playgrounds for kids, who against their parents will stay in water until they turn blue from cold and have to be warmed up on a slow fire. On a more serious note - the lakes provide a more stabile alternative swimming possibility (20+) to the sea, where the turn of tides can make a lovely 20 degree beach into a 12 degree beer-cooling tub.