According to MythBusters, bullets from high-powered guns disintegrate in less than 3 feet underwater. Slower bullets, including those from pistols, travel for about 8 feet underwater. Bullets fired at an angle stop underwater at shorter ranges than those fired vertically into water.
Water is denser than air, making it more difficult for objects to move. Forces that result in forward movement are known as thrust. Drag slows movement by pulling against thrust. When thrust is greater than drag, forward movement results. In water, drag is greater than in air, resulting in more difficulty with forward movement. The movement of currents in water also affects the ability of objects and animals to move underwater.