Even though I never have to plug
my boat into shorepower, I had a similar problem a year ago although nowhere near as nasty.
The copper paint appeared blistered around every underwater metal bit.
The problem was I had installed a large zinc to the underside of the boat and had bonded it to the bonding system.
The size of the zinc put too much voltage on the bonding system and ate the paint. Voltage between a silver electrode and the bonding system was almost 1 volt !
The big zinc went away just as fast as a small one while it did the damage.
A couple of months ago, I inquired on an electronics
message board if anyone could come up with a circuit I could build to automatically limit the zinc voltage and solve both problems at the same time without having to spend a thousand bucks.
The electronic guys were great, although it took some time to get them to understand what I was trying to do.
I wound up being served with two different schematics, easily built by me.
I built the first one and it worked great, and was inexpensive to build.
My bonding system voltage is not only adjustable to whatever I desire (-0.550 is ideal) and it's rock stable whether the tidal water
salinity goes up or down.
(We're on a salt
canal with fresh inflow during rains)
The paint problem has not reocurred.
If anyone wants to build it, here's the diagram I used.
A BIG thanks to Ron on the allaboutcircuits.com forum !