Ron,
You sure have it right with graphite grease and boats, but I think there are two different mechanisms working here.
The graphite is bad with respect to galvanic corrosion. Dissimilar metals that are separated in the galvanic series, (in the presence of an electrolyte, salt water or ionized water) form great little batteries that result in corrision of the least nobel metal. In your case it is the Aluminum alloy casting. That is why larger boats have sacrificial underwater Zinc plates (Zinc's) and outboards or I/O's have a Zinc rudder behind the prop which is also used to offset the prop walk of the engine. Zinc is one of the least noble of the metals followed by Aluminum and it's alloys. Graphite is the most noble of the metals found on boats.
The base mixing in the grease is a chemical reaction between incompatable bases that results in a reduction of their lubricating qualities. No electrolyte is necessary.
