Fighting under water with any type of weapon wouldn't really work, except perhaps a knife and magic.
Also...armor...
Best to just suspend your disbelief.
Man, I love using water hazards in PnP...
Dungeon #17 The Pit had two great ones;
- Entering the Dungeon down a ladder, there was an illusory floor with a water pit beneath it.
- Later, on an island in a subteranean lake, the boss vampire's first tactic was to hurl anyone on platemail into the water.
Best I ever saw was in Hall of the Beast Tamers in the original Forgotten Realms box set.
There were 3 chambers of pools, connected underwater, with walkways at the edge, but the Ixitxachitl Priest did not attack until the 3rd chamber. It would use wizard lock on the door, pull anyone with armor into the pool, and when low on hit points, would escape to an adjacent chamber, where it would have time to heal and rest back up to full strength while the party fished all their armor out of the pool and also healed up. Most parties would just leave after getting back to the first chamber and breaking down the door. That was a great dungeon; gorgon in the hall, rot grubs in the dead thief, an otyugh under the toilet, and the casters always blew their best attack spells on the taxidermied dragon in the lounge.
