In russia they had quite a long history of designing and producing underwater and dual-purpose guns - there is at least one pistol and 3 assault rifles... and what is their western counterpart? Only the HK P11? (+ there was a similar US pistol) Spearguns? LanceJet - underwater version of Gyrojet rocket rifle?
Or western tactics and didnt count with underwater combat?
NSWC Crane developed a number of underwater projectile solutions during the Vietnam era for the SEALs. This included a pistol similar in concept to the P11 but had replaceable cartridges instead of the disposable barrel group. Flechettes were fired from cartridges which had internal pistons which trapped the propellant gases inside. Also the Marine Mammal program was very active during this period, among the items developed was a bayonet hood for dolphins which would inject divers with gas, both killing them and sending them to the surface. A good source book would be Kevin Dockery's book, "Special Warfare, Special Weapons."