Most characters can swim. Swimming in non-magic armor requires a STR check each round. Characters swim at 1/5th of their normal walking rate, always in feet. Characters can swim vigorously for up to twice their CON in rounds, then must rest. They can tread water indefinitely.
Once underwater, a character hand hold his breath for CON rounds unless exerting himself, in which case only one half CON rounds. After this limit, the character must make a CON check or fall unconscious. Each round, +1 to the roll. Once unconscious, the character dies in CON/3 rounds.