Mythbusters is testing whether water offers any protection against bullets. Interestingly enough, so far FMJ .223 and 30-06 bullets have all fragmented upon hitting the surface of water and no done anything to ballistics gel 3 feet below the surface. I find this fascinating, and while I wouldn't want to use water to protect me (and I suspect that firing at a longer range would slow the bullets down enough for them to not fragment), it is a fascinating tidbit. They're going to fire some .50BMG later on - I'll post results then.
Any spitzer type bullet fragments when it hits a medium if it's going fast enough, and 5.56N does it better than most. That's why a little .22 caliber bullet makes such a huge, messy hole.
Do you realize how dangerous that would be?!? There's chemicals and bacteria and bull sharks in the Mississippi!