Every threat can be solved with a shotgun.

Every. single. one.

Shotguns don't work underwater.

http://i165.photobucket.com/albums/u...Piranha-3D.jpg

You just didn't buy the right shotgun.

Most modern guns will fire underwater (not sure about shotguns, but rifles and pistols should fire though), because of how the modern cartridges are sealed nowadays. You must simply completely submerge it, or else the barrel will split and be ruined (according to teh internets).

Will it fire twice? I dunno. Probably a Glock or other gun designed to fire in damp, muddy environments would.

Of course, said bullet would only be lethal for a foot or so.

That is where the auto-harpoon gun comes into play! Just find a real-life Edgar to construct it for you.