A friend from Texas has drawn my attention to the underwater assault rifle below. Every gun owner (with a swimming pool) should have one. No telling when an intruder will take a dip.
The video here and below struts the Soviet/Russian developed and still (perhaps restricted export) APS Underwater Rifle. This baby, with its 26 round magazine, fires bolts which work better underwater than conventional nose bullets. Range (5m deep) is advertised as 30m - and in the air, 100m. It also works much better than spear-guns.
Current and/or former users are special forces divers in Russia, its former allies, as well as China, India and Indonesia. Perhaps US SEALS have tested it.
Other countries make generally shorter barrelled, submachine gun or pistol sized underwater weapons like the Heckler & Koch P11 pistol. Shorter barrelled weapons do have an 2 advantages - being lighter and quicker/easier to swing around. The latter is a major issue given water resistance when more slowly swinging around a longer weapon, with a large magazine.