Do this.
1, If the old paint is still adhering in areas, leave it where it is still there. Scuff it up with 220 grit (wet).
2. Any bare spots, also scuff with 220-wet.
3. Wash the whole boat with 50/50 solution of vinegar and water.
4. Prime the bare areas with Zinc Chromate primer.
5. Apply top coat.
I use Interlux or Pettit products for the Zinc and topcoat. I prefer the Interlux or Pettit one part epoxy paints. They are a little more difficult to work with but they dry very hard and durable.
See the link below. That is one of my projects.
http://mypeoplepc.com/members/djohns19/djohns19/