"RichH" wrote
you are interconnected to ALL
OTHER BOATS in your marina

No, I'm not.
I have a private dock on the same river as the OP.
He's the one in a slip.
And I believe you have your galvanic series backwards. Magnesium is more
anodic (less noble) than zinc, which is why it's typically recommended for
fresh water. The water being less conductive, a larger electrode potential
is desirable if you want to achieve similar protection.
When I had my powerboat in the Kanawha and was using zinc anodes, they'd
develop an insulating layer on their surface after a couple of weeks, taking
them out of circuit. Abrading them down to shiny metal would restore their
effectiveness, but it was a PITA, so I switched to magnesium. This reduced
the problem, but didn't totally eliminate it, and I still never saw
significant loss of material from my anodes, which is why I suggested to the
OP that he might have power issues.