"The US Navy has deployed sea lions to detect divers in the Persian Gulf.[18] The sea lion is trained to detect the diver, connect a marker buoy to his leg by a C-shaped handcuff-like clamp, surface, and then bark loudly to raise the alarm. 20 sea lions have been trained for this at the US Naval Warfare Systems Center in San Diego. Some have been flown to Bahrain to help the Harbor Patrol Unit to guard the US Navy's 5th Fleet. Sea lions adapt easily to warm water, can dive repeatedly and swim up to 25 mph, can see in near-darkness, and can determine the direction of underwater sound. In training the sea lions have been known to chase divers onto land.

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. 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."

https://www.businessinsider.com/the-us-navys-combat-dolphins-are-serious-military-assets-2015-3?r=US&IR=T