I was watching mythbusters. They tested if water could deflect bullets like they do in the movies. They shot a bunch of different caliber rounds into the water at a realistic angle, and nearly all of them fell apart. Mind you, they shot at the water from nearly point blank range, and they determined that if you were 5 ft under the water and someone was shooting a 9mm at you at an angle, that you were safe.
50 cal, if i remember actually disintegrated into a hundred tiny harmless pieces.
edit: And this was in a calm swimming pool, not the ocean with a current, and undertow.