That is a better analysis, Chuck. But two corrections.

1. Think of the zinc and the prop as joined together electrically so
they are at the same voltage as you indicate. Current is flowing
between them. The amount of current is limited by the resistance of
the water so you can consider the water as a resistor connected between
them. This resistor (the water) also is connected to the dock, and
other boats in the vicinity so the voltage of your boat (at the
junction of zinc and prop) will be somewhere between the two. If there
is 0.9 volts available it would be reasonable to assume this makes the
boat at about 0.45 volts to the water, and the dock etc. So this
voltage will always be present across a galvanic isolator and it is
extremely unlikely you would read zero volts with a GI in working
condition.

2. You are incorrect in saying

"Regarding the operation of a
semiconductor diode, it is good to
remember that the voltage across a
forward-biased diode is related to the
current through it. If a voltage is
measured, then there is a current
through it. "

That is wrong. No current flows through a diode until it reaches about
0.6 volts for silicone. It is like a switch that won't turn on until
it gets to 0.6 volts. Once it turns on the voltage across it
essentially stays at about 0.6 independent of current. You can't
determine the current flowing through it by observing the voltage. It
does not behave like a resistor. That is how a galvanic isolator
works. With two diodes in series (each direction) NO CURRENT flows
until the voltage gets above 1.2 volts.

Regards,

Ann-Marie Foster,