In 1970 to 1980 trained dolphins killed 2 Russian frogmen who were putting limpet mines on a US cargo ship in Cam Ranh bay in Vietnam.[19] Subsequently, Russian PDSS frogmen were trained to fight back against trained dolphins. In an incident on the coast of Nicaragua PDSS frogmen killed trained anti-frogman dolphins. Arrival of underwater rifles and pistols seems to make the trained animal threat less.
Wikipedia
But the dolphins struck back.
Though the Navy declines to discuss the classified program in detail, former Navy trainers of the dolphins say the animals are being taught to kill enemy divers with nose-mounted guns and explosives.
NYT
Anyhow, look who’s interested in swimmer attack technology. Iran.
“Just a few years ago, Ukraine restarted training the animals for military purposes, though it may have been close to phasing it back out just before Russia’s annexation of the Crimean peninsula last year. In any case, Russia helped itself to Ukraine’s stable of combat dolphins after Moscow took over Crimea. And four Ukrainian military dolphins had actually ben sold to Iran and transported by air in 2000, according to the BBC.”