The distance a bullet travels through water depends on the shape and initial velocity of the round. Generally, the faster a round is moving, the less distance it travels underwater. Likewise, blunt-tip bullets retain more velocity underwater than their pointed counterparts.
Pointed-tip rounds with a muzzle velocity of 1,000 feet per second travel between 2 and 15 feet, although they are slowed enough that they are non-fatal beyond 3 feet. Similarly, pointed-tip rounds fired at 2,500, 2,800 or 3,000 feet per second will fragment in the first three feet, thus having a negligible travel distance. Blunt-tip rounds with a muzzle velocity between 900 and 2,000 feet per second can travel up to 8 feet and still be fatal; beyond that they are slowed.