Download the COC divers addon (I don't have a link on me). It allows the player to swim (but not the AI). You just need to give the player some special gear. Also, a script was made that allows the player to get out f the water and onto a boat (or something like that, I didn't really check it out). It's great fun to jump out of a blackhawk hovering low over the water and then swim silently into an enemy camp