It is common in cored boats to use plywood in areas where high compressive stress exists like under bow fittings/windlass or cleats
Either way, IMHO, accessing from the outside is a lot easier as interior
stringers/bulkheads, etc., are not in the way. Power yachts like sport fishermen are normally all cored hulls with the thickness of the outer FRG being rather thin.
Of course, if you have easy, un-obstructed access to the inner surface of the hull - access that way is preferred to be able to maintain the shape/fareness of the outer surface FRG. But normally the "strength" of the hull is determined by the adhesion of the outer skin to the "core" and the inner skin. The adhesion of the whole "sandwich" together is key to the strength of such boats.
Getting to all the wet coring or skin that no longer is bonded to core out is extremely important. Cleaning
, prepping, and installation
of the new coring and replacement of the skin so that the "sandwich" is again a single
unit is the objective. Skin that is no longer bonded to the coring will flex and "oil-can" and eventually crack and split as the boat moves through the water and waves.
Which all points to doing a superior proper job on hull repairs
. So getting good access where you can properly clean, dry, replace and compress the layers back together is not easy. Decks and cabin
tops are not so critical.