First of all, check your local and state regulations for bottom paint regulations. Your local marine store such as West's can help. The prep depends on the final selection of bottom paint. Since it is likely that you trailer your boat, look for a "semi-ablative" paint. In the colder climates you may not need as much antifouling as we need here in Florida. We use Seahawk Cucote, black color because it has excellent antifouling formulation and, it can be on the trailer and not lose its antifouling capability. For vessels that stay in the water Seahawk recommends their F-35. You can go to their web site and use an online paint selection guide that lets you set the parameters you would like. There are environmental considerations and there are bottom paints with no copper in them. I do not have any experience with them. I have used Interlux and West Marine store brand paint but all of the ones I have used have copper in them. Generally the paint manufacturers will give excellent guidance on unpainted gel coat prep. Usually it is a very light sanding followed by using their recommended hull wash. It is really a wipe. On all of the boats I have painted, I have always used an epoxy based seal coat compatible with the chosen paint. This was particularly important in the past due to blister formation on some of the earlier hull resins. I have not seen many blister issues on the newer hulls but my personal choice would be to use the sealer. Being that the sealer is a lighter color it provides a good indicator as the bottom paint "ablates." The reason for choosing ablative paint is that you need only a light sanding before repainting. I have dealt with vinyl paints that just don't want to come off after they have lost their antifouling protection. Remember, the finished job is only as good as the prep. Long winded reply, sorry Tuggers.
Pat & Carolyn, Ladybug, Too