The military navies of the world, of course, employ divers for all sorts of war time missions, not the least of which would be covertly using military-grade closed-circuit rebreathers to plant explosives on warships moored in harbors where they are presumed to be safe.
The Russian invaders recognize the danger, and according to the press reports, are deploying trained bottlenose dolphins to safeguard their vessels at their critical base in Crimea, near Ukraine. Satellite imagery from Maxar Technologies reveals two dolphin pens at the entrance to Sevastopol's harbor. The were moved there in February, about the time Russia launched its full-scale invasion.
According to the U.S. Naval Information Wartime Center, "Dolphins naturally possess the most sophisticated sonar known to science. Mines and other potentially dangerous objects on the ocean floor that are difficult to detect with electronic sonar, especially in coastal shallows or cluttered harbors, are easily found by the dolphins. Both dolphins and sea lions have excellent low light vision and underwater directional hearing that allow them to detect and track undersea targets, even in dark or murky waters. They can also dive hundreds of feet below the surface, without risk of decompression sickness or "the bends" like human divers. Someday it may be possible to complete these missions with underwater drones, but for now, technology is no match for the animals."
The Soviet navy ran several marine-mammal programs during the Cold War, including training dolphins near Sevastopol, and expanded the program after it seized Crimea in 2014. It may explain why a beluga whale was encountered by fishermen off the coast of Norway, wearing a harness with a camera mount and clip, in 2019. Scientists believe it escaped from its Russian pen and was making its way back to the Arctic, where beluga whales are endemic.
In May 2019, the Barents Observer reported that satellite images showed a secret Arctic marine mammal facility belonging to the Russian navy, with Beluga whales visible in pens. Black Sea dolphins were deployed to a Russian base in Tartus, Syria, for several months in 2018.
Of course, the U.S. Navy has conscripted dolphins and seals for its own use, having established the navy's marine mammal program in 1959. Headquartered in San Diego, it trains bottlenose dolphins and California sea lions to detect, locate, mark, and recover objects in harbors, coastal areas, and the open sea.
And in case you were tempted to dive it, the Russians are said to be using divers to guard their sunken flagship, the Moskva, even though they are not smart enough to outfox an American dolphin.