https://en.wikipedia.org/wiki/The_Day_of_the_Dolphin
though admittedly in the fiction the dolphins were trained to place the explosives and leave.
About the supposed use of dolphins in Sevastopol:
https://www.livescience.com/russians-using-military-dolphins...
they seem like a marine version of the good ol' Roman geese:
https://en.wikipedia.org/wiki/Brennus_(leader_of_the_Senones...
a sort of intrusion alarm.