How do you repair fiberglass damage to a boat?

If possible, get to BOTH sides of the damage. Remove damaged material to the point that everything is solid and "feather" the back sufficiently that you can get a good bond, using a coarse sandpaper to help the resin stick to the old material. Mix up sufficient polyester resin to coat the surfaces then lay fiberglass cloth over the damaged area and allow the resin to cure. Sand off any high spots and repeat the process until the material has been built up to the thickness of the original and perhaps a little beyond in the back, where it doesn't show. Sand the surface that shows to make it smooth. Cover it with gell-coat which has been color matched to the rest of your boat. After it cures, sand and polish again until it looks right.