Bulkhead holes
First of all put huge chamfers all around the holes on the inside and out.If the holes go all the way thru (I assume they do) then take a piece of plywood and cover it with saran wrap or other plastic sheeting. Duct tape or screw it in place over the hole on the inside. Usine WEST System epoxy and a suitable additive start laying up fiberglass matt wetting it out with the epoxy, work fast and only mix small amount of epoxy at a time, it sets up quickly. As the hole fills up start extending the layup over the chamfer area which gives the repair a lot more strength. When done Cover it with another piece of plywood and plastic so the epoxy all won't sag out of the repair. When cured (about 6-8 hours remove the plywood and do a few layers on the inside to cover the chamfer, when that cures you will need to gelcoat it or paint it and matching your existing gel coat will be very difficult. Then when the gelcoat cures sand with progressively finer paper and finish it up with compound. Good luck.