[azapa]

yes, as fondue mentions, you will destroy a pneumatic firing out of the water, the piston will smash into the piston stop unabated by the water resistance. After two or three shots you'll be looking at a rebuild.

owning pneumos and band guns I can say:

band guns are more accurate, generally because they are at a high evolutionary state re materials, triggers, guides etc, and mostly because seeing the whole spear (especially with open muzzles) give a very nice natural aim. I would say that after a little practice smallish fish (<2Kg) can be hit up to 4M.

pneumos are very powerful, driving thicker and heavier spears 5 o 6M and still bouncing off the end of the line. Even non-mamba are incredible. Shorter lengths are very possible and powerful too, great for cave or bad viz hunting. As the spear is hidden though, natural aim is lacking, and long shots are harder. BTW, pneumatic guns don't use external air sources, or loose air whilst shooting. They have an "air spring" that is loaded against and released.

My ideal comp. float would have one of each strapped on!

a bit off topic, but should clear up some of the rasied doubts.