Slow Motion AK-47 Underwater gif
Adam Rifkin stashed this in Slow Mo!
Sauce is Smarter Every Day:
They tricked me into learning science!
His Cat Physics video is also pretty awesome, especially at 3:33 ...
3-4 feet, lethal for maybe 1-2.
There are firearms that are designed to work effectively underwater, like the APS rifle but they use dramatically different designs (typically, enclosed piston ammunition and firing a long, thing, spike type projectile)
Do not EVER attempt to fire a weapon underwater that was not designed to do so (and no weapon you're going to be able to buy over the counter is going to be designed as such.)