One of the things movies always get wrong about firearms is how water will affect the way a bullet travels. Even in the otherwise-almost-too-real “Saving Private Ryan,” bullets are shown flying through water almost exactly as they do in the air: on a straight trajectory and with a lethal amount of kinetic energy. The truth of the matter is, bullets just don’t move through the water all that well. In fact, with many firearms, all it takes is a few inches of water to strip a moving bullet of the speed necessary to penetrate the human body. In most cases, water just knocks bullets off their course, but in some, hitting the water is enough to break the round apart completely. While some firearms can function under water, they tend not to function as you’d need them to if you were in a fight for your life.
This scene from John Wick 3 actually does a really good job of demonstrating how water affects bullets.
(Warning: graphic content)