OK! I have been to the boat, shore power
disconnected, here is what I found:
My ground terminal to marina power enclosure (aluminum box) = 0.6VAC/ 0.4 VDC
My ground terminal to seawater = 0.01 VAC/ 0.7 VDC
Marina power enclosure to water
= 0.6 VAC / 0.275 VDC
From Charlie's suggestion @#27, current apparently is flowing from my boat (although I don't understand how what I measured indicates this!)with a reading of .4 VDC
Marina ground to water = 0 ohms
My ground to water = infinite resistance
On my boat
Main AC breaker ground to AC ground bus is infinite ohms
DC neg bus to AC ground bus = 0 ohms
Generator/Shore power switch switches two sets of black and white wires - no bus connections and it makes no difference to voltage readings what position this is in.
These voltages don't seem high enough to say anything definite, but I will await all your welcome replies.