Arethusa- I should have stated "Any DECENT firearm" Granted a RG Rohm in .44 Magnum is unsafe both under and above water..
Consider that the pressure exerted upon the outside of the bore, by the media surrounding it will be equal to the force inside the bore, prior to firing. Hydraulicly induced backpressure would not be as great as you'd believe, as you are forcing the fluid from an open ended tube (Like a syringe, with little restriction). Also, water cavitates readily once one breaks the speed of sound in said fluid, further reducing the fluidic backpressure.
Water within a bore becomes an issue mainly when the weapon has been immersed, then the water is imporperly drained, or left in a very small bore due to capillary action. This is a circumstance where the M16 sufferd a bit of an issue in humid environments, or during river crossings. Failure to drain the bore ( "sneak" the bolt back a bit, then shake the hell out of it, muzzle down.) caused a number of burst tubes.
Supressed firearms also are not "happy" post immersion. Example? The Navy issued S&W "Hush Puppy" 39's were fitted with muzzle, and breech plugs to prevent entry into the supressor. The pistol would not suffer, but the can of the supressor would be split, or blown.
If one MUST try this, I reccomend a M1911A1 of quality manufacture. One has a low operating pressure, low velocity, and a design known for it's massive construction.
Meplat-