electrolisis
The electrolosis you're experiencing can be generated from several sources, #1: dock power console may have a ground wire rusted away, and grounding into the water, #2: the boats on either side of you in your marina may have electrical grounding problems and are exhuming electrical in the water, #3: your own boat may be expelling electrical current, in each case, if you're not carefull, Your prop and shaft and thru hull fittings etc: will start to disappear!!!, if you can't find the problem, move your boat, to another slip, if you can't, go buy a large block of zinc about 5 lbs, drill a hole in it add some yacht braid and submerge it behind your stern, and tie it off on your boat, this way the electrolosis will attack the block and not your tiny little shaft zincs.