There are boat side wires that are only hot with the ignition on, that would not involve the buzzer circuit. My recollection is that they are pink. Use a test light to probe at the connection or through the insulation to confirm hot with-key-on only. I know they are under the dash and it would not be hard to fish a wire to the transom if needed. I am less sure about locating one at the transom.Others with better knowledge of the boat-side harness might weigh in, but I think this is the right approach.
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