If the boat is indoors - a warehouse or large garage - then the main dangers are trip hazards rather than water. A plug-in RCD may be a sensible precaution outdoors on the trailer, water starts to be a problem because of puddles, bilges, condensation and rain; in these circumstances an RCD is surely a bare minimum, and a lot of thought needs to go into the socketry and keeping the connections free of corrosion on the water is a whole different ball game, due both to the inevitable motion of the boat, the proximity of other boats, and the presence of a large amount of water all around - and potentially inside - the boat. I have just installed a 240v 30A shorepower rig on my 
and this includes a bipolar RCD between the inbound socket and the 4-way distribution panel, circuit breakers on each branch (fridge, battery charger and aircon) and the 4th branch off to a line of plug sockets with individual 30mA RCD's for each of the devices which are not permanently hard-wired (kettle and chargers for vacuum cleaner, lantern, mobile phone, laptop, GPS and handheld power tools). At the present time I do not have a Galvanic Isolator, as there are no metal parts permanently below the water line, but I do have the DC Negative strapped across to the AC Ground wire, mostly to avoid fire hazards but also to protect all of the 12v devices on my DC circuit if the battery charger arcs or shorts, causing a bridge between AC and DC circuits.
By far the biggest day to day risk in a Marina is that any failure in your boat's AC rig will probably trip the entire Pontoon; my Marina Manager is not easily persuaded to come to my assistance by resetting this if the incident happens outside of office hours
I really don't think that this is overkill and I will try to take some photos next week; the RCD's are inexpensive and the most pricey piece of kit was the AC distribution panel.
