Interesting. Perhaps something like one of these on the transom?Another suggestion is NOT to run the engine.
However, there are vids of attacks where the boats have stopped the engines. So that does not disprove your theory.
I did see a rudder design ( electric I think)where the prop was mounted in the tail end of the rudder blade. I believe that it was to assist steering. Perhaps that might deter them if an egg whisk suddenly chopped all their teeth out when they went to bite the rudder