I always sand before painting. Its just insurance for yourself. Take the time and do it right the first time. Pressure wash the boat to remove dirt and debris. you'll want o WASH the boat as well. all the oils and chemicals from who knows what needs to be removed. If the coat of paint already on the boat is in good shape, then take a less grit sander and just hit it lightly. wash after you sand. let the boat dry then add a primer and paint. Some will paint directly over the original paint but whats a little time and 15 bucks of primer?
if you have aluminum showing, you'll need a "self etching primer". it can be bought at any Auto Zone or similar place.
there is no worst feeling than completing a job and having doubts. the cheapest way to do something is the right way. It might cost less $ to skip sanding and primer up front, but when your repainting the same boat every year because of a poor job, it gets expensive..
Take your time, grab a good radio and some country music

and a cold pop and enjoy your work..
Fool me once, Strike one. Fool me twice, strike three.. Micheal Scott