I have an 86 formula 242ls and I'm thinking of painting my topsides from the water line up to the rub rail. I'm going to be buying a one part polyurethane roll on paint, I'll be rolling and tipping both sides and the back, just wasn't sure on how many gallons I would need to do that.

Probably 2 gallons. Start with one and see how far the goes. I did the bottom of my boat with just over a half gallon. Also the finish on the bottom of my boat turned out good for being rolled. But did have a little more orange peel then I would like for the top.

I'd buy a cheap HVLP gun and shoot it. Rolling and tipping will be tough on verticle surfaces.
You can shoot that with under a gallon.