I carry a Fluke 1503; but, i'm not a marine el.
In my travels, most industrial applications that worry with this stuff, either a Hi-Pot test is called for, or a IR test. The IR is a better test for the threats of a boat. It is done with a powered OFF system. Shore cable disconnected.
Line and Neutral are temporarily shorted together; lets say at the shore inlet. Then, the IR meter is attached from the L/N pair, against BOAT ground. A 500V dc signal is presented across that interface, and Ohms are read. It is a HV Ohmmeter.
Lets say you want to qualify a 120 Volt system to 5mA leakage. That equates to 24,000 minimum Ohms. Less Ohms may result is GFCI trouble.
Of course, there are some hidden caveats. The first is you must disable, temporarily, the Reverse Polarity system. This can be a simple 120V light bulb glowing if N and L are reversed. Otherwise, it would introduce an unwanted ground current, causing a bad reading.
There is another, 2nd order effect; and that is capacitive coupling from L to GND, that the DC IR tester will not measure. If you had very long feeders, this might be an issue with certain GFCI triggers, if they did not take the real part of the complex current. I don't know enough about typical GFCI's to know this answer. I don't suppose it is an issue with the scale of work talked about here with small boats.
Important note: a GFCI protected system does NOT measure the "goodness" of a grounding system. In fact, in old homes with NO ground system, GFCI's are used very successfully to reduce shock risk, same as in a modern home with grounded circuits.