Well, depends, the cause of death is almost always two: drowning, or crushed by water pressure. The latter only happen when said underwater base is deep enough, and usually the cause of death is oxygen poisoning since the oxygen in your lung is pressed into lethal concentration (then crushed lungs, but the former is far more likely). The first one (which is what most portrayed by medias) can be avoided by ditching yourself outside while equipped with something that float well.
Of course, thats assuming there's no abnormal predators around you
