Why do people committing suicide by slitting their wrists always (or nearly always) lie in a bathtub of water?

Whenever I see (on TV or in the movies) or read about people who commit suicide (or attempt suicide) by slitting their wrists do it in a bathtub full of water? Is it simply for visual or literary effect, or do they really do such? And why? Surely they're not doing it to make less of a mess for the survivors to clean up (shooting oneself in the head makes a hell of a lot bigger mess than a little blood--brain matter, bone, etc.).