If you are a military diver covertly infiltrating a foreign country's military installation and you get discovered by an agent of that country (be it human, animal or artificial), you are already quickly approaching (if not across the line into) "act of war" territory.

Granted, modern international relations has done a solid job of fuzzing the delineation of "acts of war" such that interpretations of acts can serve needs in a geopolitical context, but that will be the case whether you kill the dolphin or not.