If I memory serves, I believe Mythbusters tried a 12 gauge underwater and the back pressure from the water inside the barrel caused a catastophic barrel fail. I could be wrong though....who has a pool around here....I'll bring some beer.
Only Glocks shoot under water. They also never malfunction and never ever rust.
But they have been known to melt....
So if your ever in a gunfight and run out of bullets find a pool and jump in. You'll be safe there as long as you can hold your breath