Unknown circuit - 43 Post, possibly others

Well, err, ohh,, If the shore cord is still attached to the boat and your testing the cables male end where it plugs into the dock service and ship's ckt breakers are still enabled for the ships loads, you will probably see some resistance (continuity) between the L1, L2 & neutral. The important part is none of these pins show any value to ground.

What trips the shore GFI breakers is some electricity is on the ground lead. Assuming a fault, it trips the breaker.

The usual issue with false ground fault errors, Somewhere neutral and ground are tied together in your boat, then some stray return current hits the power pole on the ground line.
The most common error on this is at the gen-set install. Next laundry machines. Next inverter installs.

Rarely, a real issue does happen; we have found a bad A/C compressor (was running great), wet outlet, wet wires and cables, smashed wires/cables and even a bad garbage disposal.
Our partner in Jax even found some bad wires in an old stove top.

The confusion and head-ache is that folks don't realize there is a problem till they move to a new dock with these mandated GFI service breakers then the head scratching begins.

A water heater could be a great example, I have heard of this issue before, just never experienced it.

Isolation transformer; Now your sounding like a Hatterass owner, don't fix the issue (that could be life threatening) just cover it up with a non ground / self generating neutral, transformer.

There was this one (isolated) boat that our divers complained about. They felt a charge under the boat and stopped servicing it. Owner never understood....
He finally left the area.

Yep, to many underwater electric events and divers want to survive.

My past crew are all still vertical when on land.
I know of other underwater events,,,, Not..

Here is the evidence that there are more issues than we commonly chat about.
SO, as my post #28, don't cover up your issues, fix them...
Stray (fault) currents have to go somewhere, a transformer is not a fix...

240Vac Isolation transformers do not use neutral (white wire) from the dock service.
Isolation transformers may not connect the ships ground (green wire) to the dock service either.
The dock service does not receive any fault current and the GFI service breaker does not trip.
Using this approach, any fault current stays on the boat, tripping breakers or leaking to the water.

While using a gen-set, neutral and ground (white & green wires) are strapped together at the gen-set.
Here again, any fault current stays on the boat, tripping breakers or leaking to the water.