Methane released under water rises to the surface in great bubbles, sometimes destroying a ship that happens to be in the wrong place at the time. Other gases released under water do the same thing.

Not only is this the reason that people who work in places that ozonate water for drinking are forbidden to wade in the tanks, but why you should never get into a swimming pool with a bunch of four-year-olds (well, there's another reason too, but that becomes part of the the pool water).