Re: painting a boat
Except for once when a friend and I wanted a truly perfect paint job, I've always used the gelcoat or the previous paint as my primer. As long as the original coat is still in good condition (aka not peeling/cracking), then I just scuff sand it and paint away. When you do this though, you do have to make sure that the new color will fully hide or blend in with the old. For example, you aren't going to have much success painting white over a dark red.
Best of luck though, nothing makes you more proud of being on the water like a fresh paint job and a new interior.