This will sound like a joke, or out of the Red Alert series, but the US does literally train dolphins and other sea mammals to patrol Naval bases, search for naval mines, attach torpedo homing beacons to enemy ships, aid in search/rescue and equipment recovery...

https://en.m.wikipedia.org/wiki/Military_marine_mammal

I do not know of a recorded instance of an actual man on man underwater fight... it would likely be absurdly classified by at least one, if not both sides.

Frogmen usually do things like sabotage of underwater assets like infrastructure (internet trunk lines, oil and gas pipelines) or plant or remove sonar beacons... or infiltrate into enemy territory to then later sabotage something just under the water line, or a bit inland.

The aggressor would not want to admit they were there, and the defender would almost certainly not want to admit their security perimeter had been breached, or would be unable to definitively assign blame unless they had very conclusive evidence, like a dead or captured frogman, or some kind of unnassailable, traceable materiel remnant.

Most aquatic guard units would be attacking an underwater commando team from the surface, using some kind of non human detection method to search for and locate underwater aggressors.

AFAIK nobody really maintains like a 24/7 underwater patrol of human divers to patrol sensitive areas. But I could be wrong.

I guess uh, email Jesse Ventura rofl, maybe he knows more?