There's no weapon quite like a school of pet piranhas.
The Roxy Surfari Sidewinder Bikini Bottom might not be very good at stopping bullets, but the Sidewinder is in its element once you get to the waterfall and jump in. Whew, you lost the gun-happy bad-guys. Now there's just the huge dude with the metal shark-teeth to deal with. It might be a good time to use your piranha-charmer flute.