[fillet1]

First you will have to clean out the damaged area. Is the gelcoat chipping up? If so, you have to dig out all loose material. The area will have to be rough sanded and cleaned prior to repair.

Use West systems epoxy to repair the glass below the waterline. They sell it at any boat store. Its the best for waterproofness. After fully cured, sand, and apply gel coat over the cured epoxy. Sand, wetsand to 1200 grit, compound, buff, wax and you are done.

If your boat is bottompainted then just epoxy the thing, sand it smooth and repaint with antifouling.