There are many reasons that muscles cramp. Of course, some are medical and a visit to your physician may be appropriate. However, cramping can come from over fatigue during exercise, exercise in a cold, wet environment (pool?) or weakness in the cramping muscle. There are also nutritional reasons muscles may cramp. Swimming is great exercise, but mostly for the upper body. Strengthening the muscles of the hip and knee can improve exercise tolerance in the pool. Eating a well rounded diet with plenty of fruits and veggies should insure the proper nutrients muscles need for good functioning. And morning swimmers should eat a light breakfast an hour or so before hitting the pool. Running the body's engine with out fuel can also cause cramping.

Good luck.