When you measure that potential Bill R suggests, be sure you measure it both as AC and DC voltages. If you find a significant AC voltage between the windlass and earth, I would unplug from shore power right away, confirm that the voltage has gone away, and stay unplugged until the problem is diagnosed and fixed.
If there is any kind of cross connection between an AC "hot" wire and the DC system, or the bonding wires it has the potential to be quite dangerous.
