[quote=Raxacoricofallapatorius]
While it may be water proof it may not be air proof, being underwater is significantly different to being in a vacuum with no atmosphere at all, and all the air will likely be sucked out and dissipate into nothing.
[/quote]

I do believe you are missing the point:

The oxygen for the reaction is contained within the powder itself. The reaction does not rely on oxygen in the atmosphere at all. If it tried to? It wouldn't work, because there is not enough volume in a cartridge case for both sufficient powder, AND air, to operate at the same efficiency.