Both. I moved to a different slip about 50 yards away and corrosion is the same. It stops when off shore power so the ground isolator should work/help. I only go on shore power during the winter to use heaters.
Unrelated issues, and no it doesn't make sense.
Almost all of the galvanic isolators built after about 2005 are 'Fail Safe' meaning that if they fail they will still allow AC current to flow on the grounding conductor but they may not stop the DC current that causes corrosion. It's usually only lightening strikes that causes them to fail. You can test it by using the diode setting on a multimeter and checking across the disconnected galvanic isolator and you should see the 'breakover voltage' of the diode, around .8 to 1.2 volts, which tells you it is still functioning. You will see voltage rise slowly as there is also a capacitor in the circuit, but you'll see zero ohms or open if the GI has failed.
I looked at your suggested location (nook/cave next to AC panel). I have room. Is that entire panel with all the monitors and controls removable?
I would worry more about getting it in the right position 'electrically' than its physical location. Like some of the others mentioned I put mine in the battery compartment, electrically it was in between the shore inlet and the ELCI breaker. The important thing is that it's in the AC grounding conductor BEFORE that conductor is tied into the DC negative system, so the closer to the shore inlet the better.
I agree. Getting to the shore power ground cond from the STBD lazarette is a pain. If I can remove the panel with the AC panelboard, I’d have easy access to the ground conductor. That’s where it goes/terminates. Once the cond is cut, I can run the spliced conductors and mount the isolator anywhere.