Currently, my gaming group are making their way through an underwater temple with large portions of flooded corridors and chambers. I’m also running the game using the Adventurer Conqueror King System (ACKS), which does not (as far as I have seen) no real rules for underwater combat; so I made some up on the spot:
Basically, each non-encumbered character can fight or move 10′ for 1 round underwater per point of Constitution. Each point of AC reduces this number. After the rounds have expired, the character begins to drown, reduced to half hit points in the first round of drowning, to zero hit points in the second round, and dying on the third.
For example, a dwarf with 15 CON and wearing plate mail (AC 6) can cope for 9 rounds before he begins to drown.
Nice and simple, and worked well last session.