The fitting on the end of a spear was/ is called a bang stick or power head. It comes in a number of calibers, from 22LR to 12 gauge shot shell. It was designed to use against a shark attack and now is used for gator hunting. While you could use it against a diver, we never had these in the teams, nor spear guns. But a revolver can be carried under water and is effective if used right up against an enemy or even a shark. At 3-6 feet, the bullet has lost all power.
https://www.billsbangsticks.com/