try running the engine in total darkness to see if any high voltage is jumping out of it’s assigned path ( plug wires, plugs..whatever)

you can get zapped just rolling a kart at walmart by static electricity build up from rolling wheels …getting zapped means you are becoming a component in the circuitry to ground !

is the engine mounted on wood / non conductive boards sandwiched on the transom…? try running a test ground wire between the engine frame and the aluminum of the boat

you need to provide a return path to ground other than via you body !