That's right, some cartridges, such as some teargas and breaching rounds do not develop sufficient gas pressure to cycle the mechanism.
I'm not sure, but i think firing a weapon underwater is wildly stupid, because the barrel won't take the pressure, since water has higher density, the pressure in the barrel would rise and the barrel would explode. Anyone ever heard of ppl who got snow in their barrels? Anything that blocks the barrel, can cause the pressure to get to high, because the bullet won't move fast enough and might cause explosion...