The honest answer first: an RCD trips because a small amount of mains current is finding a path back other than through the neutral conductor. On a boat that path is usually damp, and it is the same path a person could become part of. The device is doing its job, and resetting it repeatedly until it holds is the one response that must never be adopted.
There is a second possibility worth separating early. A breaker that trips instantly under no load behaves differently from one that trips after ten minutes with the water heater running, and the two point at different faults.
An RCD monitors the balance between live and neutral and disconnects when a few tens of milliamps go missing. A miniature circuit breaker instead watches total current and disconnects when the load exceeds its rating. Marina pedestals commonly carry both, and knowing which one operated tells you where to start.
If the supply drops the moment you plug in with everything switched off, suspect leakage in the fixed wiring or the inlet. If it survives until several appliances are running, you may simply be exceeding what a 16A pedestal will give you, particularly with an immersion heater and a fan heater on together.
Leakage faults on boats cluster in a small number of places, and they are almost all about water finding an enclosure it should not be inside. Salt residue makes matters worse because it stays conductive long after the surface looks dry.
The one safe test an owner can run is elimination by isolation. Unplug every appliance, switch every AC circuit breaker off, then restore the supply. If it holds, bring circuits back one at a time and then appliances one at a time, and note which one drops it. That gives an electrician a starting point and often saves an hour of diagnosis.
Swapping the shore lead for a known good one is also worth doing, because leads take more abuse than anything else in the system and a damaged one will trip a healthy installation.
Some causes sit in the fixed installation rather than in anything you can unplug. Neutral and earth bonded together somewhere on board, an accumulation of small leakages across several circuits that individually pass and collectively trip, and deteriorated cable insulation in a bilge run all behave this way.
These need insulation resistance testing with proper equipment. Reading them requires knowing what a marine AC system should look like, which differs from a house because of how the earth is handled between the pontoon and the boat.
Mains voltage on a boat is a serious matter. You are surrounded by water, standing on a conductive structure, often in a confined space, and the protective earth arrangement is more complex than anything ashore. A trip is the system telling you the protection is working; a bypassed or persistently reset trip is an electrocution and fire risk with nothing left to catch it.
Do not disable the RCD, do not use an adapter to defeat it, and do not run the boat on an extension lead arrangement to avoid it. Get the installation tested. We carry out shore power fault finding and testing along the South Coast, and where the fixed wiring is at fault we can put it right at the same time.
Once the cause is fixed, most of the recurrence risk sits in maintenance. Keep the inlet sealed and its cap on, stow the shore lead dry rather than coiled in a wet locker, and have the AC installation tested periodically rather than waiting for it to announce a problem.
If your boat also suffers from stray current or galvanic issues in the marina, addressing the earth arrangement properly with an isolator or transformer often improves both problems together.
Try a different pedestal with the same lead. If the fault follows the boat, it is on board. If it stays with the pedestal, tell the marina office rather than continuing to reset it.
No. On an overload that would allow cable to be run beyond its safe capacity, and on an earth leakage fault it changes nothing at all, because the RCD is measuring imbalance and not total current.
That pattern strongly suggests water ingress at the inlet, at a deck fitting, or into an exterior socket. It is a real fault, not a quirk of the weather.
Usually the element has failed internally and is leaking to the water it heats. Replacing the element normally clears it, but the installation should still be tested afterwards to confirm nothing else is contributing.
We test AC installations, find leakage faults and repair the fixed wiring, with no reset-and-hope shortcuts.