If your boat is made of polyester, why not just do the repair with polyester resin, fair it with a polyester based fairing compound such as poly fair and then use a polyester based spot putty such as ever coat for the many pin holes and then gel coat it. The trouble with doing a repair with gelcoat is it takes a lot of work to get the gel coat like a factory moulded finish. Theres plenty of wet sanding involved, and then applying more coats of gel coat, its a tedious process. We`ve found if you thin the gelcoat out with duratech`s clear it can be sprayed fairly flat and shiny, but the duratech clear coat will break down after expousure the the sun.
As for the jet-ski?( get ski ) , the same above applies. The easiest thing for you to do is sand the bottom with 150 - 220 grit, and then find a supplier that deals with awl-grip paint and expoxy primers. This paint is fairly expensive, but as anyone here uses it knows its durability. Other than that, find a sand your hull, find a good expoxy primer, and get some good quality automotive paint, emeron works great,international make a good paint also for the marine industry. But you gotta prime the get and sand the gel with 150 - 220 grit, if not the paint will peel right off.