It makes crazy when people just reply to tell me to go someplace else to read about my question but...I apologize.. Look up galvanic corrosion It can be caused by your boat, possibly by your marina if the wiring to the dock is old and in disrepair, or other boats near you wired wrong.
I wired a Galvanic Isolator into my shore power system and hope that it is working as it should.
If your shower power cable is going directly to your battery charger only I would guess the biggest worry would be the marina and boats next to your putting stray current in the water.
Is your shore power green going to your boats common ground?
check this out
http://www.yachtsurvey.com/corrosion_in_marinas.htm