Most boat builders do not build boats with epoxy resins because it is too expensive (unless it is a cedar stripped canoe, etc.). Boat builders use polyester resins (usually isothalic in nature) and some use vinylester resins, which are an epoxy modified resin. The early sailboats were made with orthothalic resins (also polyester) and after some time they realized they were more susceptible to blisters. So, that is why they went to isothalic. When I do fiberglass repairs - especially above the waterline, I usually use polyester resins and Evercoat polyesters are fine for marine use. Epoxy is very expensive in comparison to polyesters, but it is very waterproof, so it is good for underwater repairs and also adheres to metals and wood better (flexes better with wood). Epoxy has a compatibility problem with gelcoat. Polyester does not. So....my suggestion is to use polyester resin in most of your repair jobs. It is also good under the water line if you put an epoxy water barrier paint over it. And, below the waterline alone, I think you will find it will hold up quite well. I have repaired more boats than I can count with polyester resins and have used more gallons of the Evercoat product than I could ever count. Polyester will cure quicker than epoxy too, so your job can get done faster.
As far as the actual repair goes, I need to know more about the damage, but basically you want to use a lot of cloth or mat and properly wet down the fabric with resin so that you get good penetration. Your strength comes from the fabric. If you use too much resin and not enough fabric, you will compromise the repair strength. You can use Evercoat polyester putty to smooth out your repair prior to the gelcoat finish too. They make bondo for cars (usually red) and they make a marine formulation (usually an off white). Use the marine because red is harder to cover up with white gelcoat. Also, don't use too thick of putty as it has little strength in comparison to the cloth/resin matrix. Too thick of putty will eventually crack. It's like using concrete without rebar.
If money is no object and you go with epoxy, be aware that measuring must be exact between resin and hardner (per mfg recommendations) while not so critical with polyester (but try to stick to the directions). Also epoxy is not good for you and you must protect your skin and lungs from breathing it. If you get senstitized to epoxy you will not be able to go in the same room with it while polyester is not like that at all. Always wear gloves, etc. and don't let epoxy touch your skin. It will take a while to get senstitized, but if you do, you will regret it big time.
If your hull was built with vinylester (which it wasn't), then I am saying that using vinylester resin is a good idea. Some mfg's will use vinylester under the waterline for blister prevention. Rarely will you find a composite boat built of epoxy unless it is a very expensive high tech performance vessel, etc.
