Get a fibreglass repair kit (David's) from an auto shop or chandlery. Grind back around the hole and make certain it is dry. You need something solid behind the hole, so if you can't get access behind cut out some cardboard and line it with cellophane and attach a string through it. Pull this board forward to give you a surface to lay the glass against. The rest of the instructions will be on the kit, place the matting down sheet by sheet and add the resin. You can get a gelcoat filler or marine epoxy to finish the job nicely with a sanding.