Couldnt get that approach to work.

But then looking at the bottom of my boat when recently dried out in Combe, there was very liitle erosion of the ally anodes and no sign of any electrolysis. The zinc anode was covered with a thick white film. Thats alltogether much what I would expect - I dont leave the leccy plugged in when I leave the boat, and physical chemistry says that the electrolysis problem will be much less with a much lower level of salt in the Bay water anyway.

So in short, I reckon the issue is what anode can you use that will protect you when outside the bay without being made useless by being parked most of the time in fresh water. Ally seems to be the answer.