Hmm.

Seems to me GFCIs pop if there is a difference between hot and neutral
wire currents. Don't rely on ground. If all the current that comes in
doesn't return the way it's supposed to, it turns off. It's a
differential amp that does it and toroidal coil assemblies. The test
button might need earth, so it can trip the circuit by cheating. Proper
ones don't.

The Earth should not be electrified, nor the Sea neither. It's bad
enough that we might inadverdently offer the possibility for current to
flow into the earth by touching a live wire, but if the power company
hadn't hooked the earth to the power line, it's wouldn't have any where
to leak to whilst electrocuting you.

Aboard, if metal parts are earthed, earth the plug, otherwise, it
don't matter. Sea water in the bilge sort of grounds wet stuff, but
it's moot.

There are several possible faults that are best protected by
contradictory means. It's a question of probability and risk
assessment.

It's your boat. Read your insurance documents for electrical
requirements, survey report required? Marinas don't seem to care. Old
boats may be grandfathered. Fire is the real danger.

Terry K