Correct..
Finally a subject that combines my professional job (gunsmith) and my hobby (diving).
The Glock 17 was originally the one designed to be fired underwater, under certain conditions. It has been rumored that the other models will, but I wouldn't do it.
This can only be done with the maritime spring cups installed, special, waterproofed, non-expanding ammunition (we don't want hollowpoints expanding in the barrel, do we?)
The range of the rounds fired is severely limited. Probably (I'm guessing) would be limited to 5 feet or so, with the energy of the round fired dropping FAST.
As ereediver mentioned, the most important aspect of this is your eardrums. Upon firing the first round, both of your ear drums would explode, REALLY wrecking your day. Earplugs? Not much help for a diver, can't wear them underwater anyway.
Advise? Don't do it.
So, the moral of the story, your Glock isn't the preferred weapon of choice when subduing charging, rabid tanks underwater.
Darin