If it's been in water with that much damage you could be looking at a bit of osmosis in the fiberglass (F/g cancer).

Best bet is to take a fairly wide cut around the damaged area and replace the section completely with three laminations, the outer one onto the scraped down outside/middle of the fg ie undercut by 1/16" deep x 2" wide around perimeter to retain outer profile, the middle layer to the remainder of the thickness of the fg from the inside, and another layer overlapping the whole area by 4" from the inside.
____________________________
........| New outer
.Old...|____________________
......./...2"....|
.................| New second
___________|______________

New third
_____4"____|_________________