The question isn't if it is possible, but why would you do it?
Because you never know if your big, fat, hairy next door neighbor might decide to jump in the tub with you while you're scrubbing with Mr. Bubble and playing with your tub-toys.
I've often wondered this too.. I mean I know there is a spec op need for it and the russkies have all KINDS of cool underwater shooters.. but I'm no abalone diver and the idea of shooting up my swimming pool to test the ability of my eardrums to burst well.. that just seems like a darwin award waiting to happen.
Isn't the effective range of the bullet underwater measured in inches? I heard somewhere that a bullet underwater will go about 3x (??) its penetration value in gelatin?
In other words, a load that penetrates 12" in gelatin might go 3 feet underwater.
...I told my beau that he needs a bayonet for his 590A1. He looked at me like I was nuts and said "What do I need a bayonet for?".
I replied "Well, you have a bayonet lug, don't you? There you go."