if any part of your body goes into cramp stop swimming instantly, if u get it in your thigh bad enough it will snap your femur in 2, and then your fucked, dont try open water swimming in seas, find a lake and practice doing small distances at first, its basically like asking a guy who bench presses 10kg to go against a guy who does 300, major damage and as people have stated it can be very dangerous, also look into learning how to control your body temp, some guy just swam the north pole, he heats his body up by a degree in less than a min and keeps it there