Tell the admiral to check the fish dear, your engines sound unique, all engines sound unique as you know, so its only going to activate when your passing by - you place a low power transponder within a muscle, or deep sea fish, that activates upon sonar resonance.
I would strongly suggest to EM the waterway and check the international cables.
If they used to run submerged fuel in the cold war, what makes you think they wont run submerged short wave sonar - old tech, low tech, no one is scanning for.
Just my 2 Satoshi