It has recently emerged that the Russian Navy used specially trained marine mammals, probably seals, for harbour defence in Syria in 2018.
This is part of what has been a very long lived program by the Russian Navy. Dolphins are trained in anti-diver warfare, seen here in Sevastapol harbour.
The marine mammal deployment in Syria lasted several months, in conjunction with the employment of several anti sabotage and harbour defence vessels.
The unit these animals come from has an interesting history, it was established in the 1980’s and based near Sevastapol. This became part of Ukraine after the breakup of the USSR and was integrated into the Ukrainian Navy. However the unit became inactive, only operating as a tourist attraction.
After the Russian annexation of Crimea in 2014 it was reintegrated into the Russian Navy. It has used dolphins in the past and may use seals.
During the Cold War both Russia and the USA developed capabilities in dolphin based warfare. These programs used to be top secret, over time more information has emerged. This video provides a good overview of the programs.
The US Navy has been quite open about its sea lion unit. This video shows them at work in Bahrain. They are trained to detect mines, swimmers and divers that may try to infiltrate a port.
The US Navy dolphins demonstrated their skills recently when they discovered a 1880’s era torpedo while training.