Somewhat depends on how you're going to apply the paint. If you're going to spray it, I won't make any suggestions, as I have no experience doing it. For brushing - or more accurately rolling & tipping with a 2-part - I'd be looking at either AwlGrip or AlexSeal. AwlGrip's brushing reducer does wonders in maintaining a wet edge while you're painting. I've heard that the AwlGrip brushing reducer will also work w/ AlexSeal.
For a 1-part, I'd talk to a reputable paint dealer about an industrial enamel. Color options may not be as great as "boat paints", they typically are very durable. Built & painted a boat for a guy who wanted it red. At the time, I couldn't find a viable red boat paint, so used "industrial red" (fire hydrant paint). It outlasted everything else the guy did to the boat.