This Pistol is a weapon for those who frequently find themselves in the water. No amount of immersion will make the powder loaded into it wet. It will even fire underwater as well as it would on dry land.
Unless the wielder has some way of keeping their other powder dry, then it will be good for only one shot, as only the powder in the gun is kept dry, and it doesn't magically dry out wet powder that is loaded into it.
Even so, in many situations, one shot might be all that you need!