https://en.wikipedia.org/wiki/Powerhead
A powerhead, bang stick, smokie or shark stick is a specialized firearm used underwater that is fired when in direct contact with the target, not to be confused with other types of underwater firearms which fire specially designed projectiles and do not require direct contact with the target. Powerheads are often used for spear fishing and against sharks or alligators for sport, defense, or to kill nuisance animals. The term powerhead refers to the firearm-like part of the device; when attached to a shaft to form a spear, it may be referred to as a bang stick or shark stick. The spear in question may be handheld or launchable from a spear gun.[1][2][3]

https://en.wikipedia.org/wiki/Underwater_firearm
An underwater firearm is a firearm specially designed for use in an underwater environment. They are found in the arms inventories of many nations. A common feature of underwater firearms or needleguns is that they fire flechettes instead of standard bullets.

https://en.wikipedia.org/wiki/Flechette
A flechette /flɛˈʃɛt/ fleh-SHET is a pointed steel projectile, with a vaned tail for stable flight. The name comes from French fléchette, "little arrow" or "dart", and sometimes retains the acute accent in English: fléchette.