Actually, most normal pistol rounds break up on the water's surface. A rifle round which is slightly pointier, thus allowing for penetration, will go through the surface. Most submachineguns also use pistol rounds, and even a higher count of bullets won't break the surface.
Actually that's almost completely backward. Slower pistol rounds penetrate much further than faster assault rifle rounds. Anything above a .45 tends to turn into fragments an inch or so below the surface. Water doesn't compress, and the metal can't take the friction/force impacted upon it when it's moving that fast and hits the water.
If you want to hit someone in water use a 9mm, rifles just create slow, fluttery shrapnel.