This is a very common problem. It can be caused by fins that are too stiff and offer too much resistance. This can be remedied with a split fin that will give more. This will allow you to enjoy the dive without those agonizing leg cramps. If you get leg cramps while in the water you can always extend your leg straight and grab your fin tip and pull it towards you. This will give you a a good stretch and alleviate the cramp.