Link to
http://www.pencilstubs.com/magazine/MagPage.asp?NID=147 and search underwater gun.
Sounds like a waterproofed variant of the gyrojet called the "Lancejet" or perhaps the Underwater Defense Gun Mark 1 Mod 0, as the more common Heckler & Koch P11 didn't have spin-stablised projectiles.
--- Steve K9ZW via SPOT BBS
--- Mystic BBS v1.12 A48 2022/02/11 (Linux/64)
* Origin: SPOT BBS / k9zw (700:100/69)