Well I've replaced all of my 1989 vintage outlets on my 1989 OA 44' CPMY with GFCI's and what I found is that as some have mentioned the standard home brew GFCI's are sometimes too deep for simple replacement. But the most difficult issue to overcome is that the home based type of units don't allow for complete removal of the wiring screws. This means that if you have ABYC approved wiring terminators (ring connectors) you may have to do some terminating with the captive type of spade connectors (the ones with "hooks on the ends") of course if the manufacturer left precious little in terms of length of cable it can lead to running a whole new wire. On my boat ALL outlets are run individually to each outlet so no installing a single outlet that supplies GFCI to outlets "down stream". If I were to do this again I would opt for the "Marine" versions just to avoid this problem.
RB Cooper