Oh yeah, I had an idea once for a flamethrower disguised as a fire extingusher. It would only be good for use as a last resort against a boat that was ready to board yours and would make quite a mess of the opponents.
Best not to get confused in an emergency!
It would probably be a better idea to disguise it as a "damaged" scuba tank, or something smaller like an emergency O2 tank, with a fast attach nozzle of some sort stored conveniently nearby.
Or why not simply make one out of an old spray paint system? That's something you might easily have on board a boat. Heck, you work it right and you could still use it as a spray paint gun.