Some current leaks to the water from any boat on shore power. With a fault it can be enough to kill a swimmer near the boat, which is called electric shock drowning. An ELCI is a breaker on the shore power that compares the current going out on the hot wire with the current coming back on the neutral. If they differ by its threshold, the rest is leaking, and it trips. In the Blue Sea Systems diagram Panbo uses, that's 30 milliamps.
It works like the GFCI by a galley sink, but for the whole shore supply. Panbo notes the latest ABYC standards and the US electrical code call for one both at the marina pedestal and on the boat. Boating notes it doesn't protect the wiring between the shore inlet and the ELCI itself.
What a surveyor checks
Whether one is fitted at the shore power inlet, and that it trips on its test. Signs of trouble in the AC system: nuisance tripping, an inverter that ties neutral and ground the wrong way, which Panbo calls the most frequent cause. The inlet and cord for water and burning. On older boats, none fitted is normal and gets noted as a recommendation.
In the report
"Shore power, no ELCI fitted. Recommend fitting an ELCI at the inlet, after the boat's AC leakage has been checked." See ABYC E-11.