Thanks for your responses.

For Gordon: the RF ground/counterpoise is a copper foil which - I
think - is connected to the engine block.

For Chuck: I've not noticed the same erosion of the anode when in any
of three different marinas and connected to shore power, including for
months at a time. I've only noticed the erosion after passages that
have included daily Pactor modem work. I renewed the shaft anode 3
months ago, found it erosion free 6 weeks ago before leaving a marina,
and found significant pitting when I checked it two days ago after
entering a marina (with 6 weeks of passage making and no shore power
connection in between checks).

Bil