Also, protagonists who have to make a rapid ascent from any significant depth of sea water, especially to escape a shark or other similarly threatening aquatic resident, will never suffer nitrogen narcosis, nor have to stop and equalize lung pressure, etc., etc.
Addendum: one of the worst things you can do in movie land is board a submarine. It's almost certain to spring a leak, suffer a depth-charge assault, hit the ocean floor, or be irradiated by a breach of the nuclear core.
Warrior Poet