Interesting.

Stainless steel I understand will corrode - big time. That is why stays have to be replaced. It is iron, and, as my research has shown, corrodes just like iron, except it doesn't turn brown but turns greyish. It also depends on the stainless steel mix (304 v. 316)

There is no question that there is significant galvonic action, even in fresh water. I have had the bronze filter plate over the head water intake totally eaten away. That was when the water intake was the ground point for the Ariel electrical system (standard Pierson issue). There is no question that there is substantial corrosion occuring around my outboard (which has a generator and is connected to my electrical system), even with a zinc plate, which appears to be getting smaller at a significant rate. The prop and the lower unit are eaten away unless they are liberally covered with non-electolytic paint (Tri-X for example). These parts are all connected to the electrical system, and to the mast head, which is a major source of potential differential and, hence, electrical current.

Those are emperical pieces of evidence.

I don't appear to have experienced any corrosion on a) the copper grounding plate a couple feet under water, b) the rudder shoe or shaft or c) the head outlet valve.

I berth the boat at a slip but do not have slip power (except when charging the battery a couple times a season)

Crazy, eh?