"Hey is that inside a fish tank?"
"Is that a underwater rifle?"
Yeah, basically what I found is the Russian ADS underwater assault rifle designed for use by combat divers. Pretty neat huh? Here's its wikipedia link. The picture is a ADS at an arms show, inside a "fish tank". Definitely about the best way to show off a underwater weapon.
As you can see, it has a grenade launcher, but there aren't any underwater-use grenades designed for it. (as far as the Russians have mentioned, mind you).
So overall pretty neat. - Andrew