[mauricefrompolo]

Thru hull deterioration as well as anodes is a by product of having shore power connected to your boat "without" the use of a galvanic isolator. Normal routine is to replace any suspecious metal work below water at haul out time. The reason that the surveyor probably could not find a reason for the stray current is the fact that this is generated by any and all boats in close proximity to yours that are also connected to shore power. Salt water is a much better conductor of these stray currents than fresh water. Have him give that a go.