OSB is an awful choice. Marine grade plywood would be best, or at the very least a good grade of regular plywood.

You can paint it with porch paint, but carpet will give you a nicer result as I've been there and done that with several boats before. Carpet isn't that expensive, and fairly easy to replace. The closed cell EVA foam is kind of the way to go now when it comes to an alternate for carpet. SeaDek and HydroTurf are pretty popular, but there's plenty of off-name brands. The EVA foam feels great on bare feet, and is easily sprayed off. There's all kinds of colors, even camo that hides dirt pretty good.

If money is the problem and incredibly tight, I would go to Tractor Supply and buy a rubber stall mat for $40 and cut it to fit in the boat before I would ever do the porch paint thing with OSB. A rubber stall mat is 4 ft x 6 ft and 3/4" thick, it's heavy enough that it won't blow out and will just lay in there without glue, you might not have to put plywood under it depending on how your boat floor is. Only drawback to the mat is that it's black and will get hot in the summer sun, but it is cheap, especially when they put it on sale for about $30 at various times of the year.