wvmarle:
Just use Bluetooth or any other RF dongle that can handily reach your required distance. Get below deck and the extra material in the way reduces the range, you don't want it to be so sensitive that it shuts off because of that.
RF doesn't work under water (the water blocks the radio waves with in a few mm), so the moment you go overboard even right next to your engine the contact is lost.
Brilliant solution to a very xy problem.