Mythbusters did an episode on this showing that movie scenes of shooting into the water and killing people who were submerged was just about impossible.
I remember. Even with a 50 Cal sniper rifle any thing more than a foot or two underwater would likely be unharmed by the projectile.
IIRC, the Mythbusters episode showed that high velocity rifle bullets fragmented on contact with the water, but lower velocity shotgun slugs and pistol bullets retained lethality a few feet down.
I remember that one.
What was amazing was the 50 cal. Sniper Rifle had no depth penetration of the water at all. The round just disintegrated into hundreds of pieces and dropped to the bottom when it hit the water. They said it was due to the energy of the round striking the water which destroyed the bullet.
They found the lower the velocity of the round striking the water the better it did traveling after entering the water.