Have you not watched the Myth Buster's episode where they actually fired guns under water?
As long as there is no air in the barrel or chamber, the gun will fire - revolver, automatic hand gun, bolt action rifle all fired under water.
Careful, Jim may yet find a use for his gun!
Please note that I just commented that it's
not recommended.
Results confirm that:
http://kwc.org/mythbusters/2006/04/e..._reopened.html