Thanks for the comments, Ann-Marie.

Andina Marie wrote:
Chuck, in over 20 years working on boats I've never encountered a boat
in the water on shore power that had zero volts across the Galvanic
Isolator. Just the zincs on your underwater metal will introduce about
0.5 volts DC.

I don't see how the zincs on your boat
can cause a current to flow through your
GI (and thus the shore power ground). In
the common case of a bronze prop and a
zinc on the shaft, for example, the
galvanic current passes through the
shaft. It does not pass through the GI.

A neighboring boat using the shore power
ground to complete a galvanic circuit
with your zinc would definitely cause a
current to flow in the GI.

Even if the neighboring boat was
protected with its own zinc, there could
still be a small galvanic couple created
with your zinc, especially in sal****er.
Not nearly as likely in fresh water.

Other than that, I would have a
difficult time explaining the voltages
you measure.

Chuck