[Gerry Zagorski]

Do you keep your boat slipped in a Marina D?? If so it might be caused by one of your boat neighbors or the dock wiring introducing stray electrical currents into the water. This is usually caused by a boat or the docks not being wired correctly.

Typical culprits are people using standard 110 3 prong plugs rather than shore power cords to run their battery chargers.

You might also make sure your boat bonding system is working correctly. If the rest of your zincs are not showing ware, it could be that the bonding wires to those other zincs are not making good contact so the stray current is attracted to the zinc on your tab that is bonded correctly.

I would check all the bonding system connections and make sure they are not corroded. They are typically green wires that run to anything metal below the water line and are all run back all to one comon place to ground them. Check the connections on both sides.

I would also see if other boats in your general area are having the same issue you are... If so then it's got to be a another boat in the area or an issue with how the docks are wired.

These sorts of problems are tough to find and your best bet is to make sure your bonding system is in good order along with changing the zincs when needed. You have to be very diligent about finding the issue or replacing your zincs. Had a friend of mine who was having an issue he left unchecked a few years back... By the end of one season, he ended up having to replace both his outboard engine mounts because they got eaten away so bad.