-- Last Updated: Oct-13-12 6:48 PM EST --
If you are going for a really nice cosmetic finish marine polyurethane paints will probably be slightly more durable but any spray paint will work.
Some paints are claimed by their manufacturers to be specially formulated to bond better to plastics (such as the vinyl of Royalex). One such is Krylon Fusion. I believe that the Fusion formulation bonds to Royalex a little better than their other paints, but others don't think so.
Daggermat has had trouble with the red Krylon Fusion spray paint clogging before the can was empty. I have used it, and not had that problem, but I have never sprayed on an entire can at once and frequently shake the cans during use, and always invert the can and spray to clear the nozzle after use.
Be aware that any paint, even 2 part marine polyurethanes such as Interlux Perfection, are subject to scratching. If I was in your position I would consider painting the hull above the waterline with a one part marine polyurethane like Interlux Brightsides. This is easily applied with a foam roller and "tipped out" with a disposable foam brush and gives a very nice finish that looks as if it was sprayed on.
Since the hull bottom will inevitably get scratched, I would paint the bottom with a readily available spray paint like Krylon Fusion. A light color would probably look nice. Put the boat on a perfectly flat surface and cut a stick 4" in length to use to go around the perimeter of the hull and mark a 4" waterline. Mask that off and spray it. You would probably need a couple of cans for the bottom.
When the bottom becomes scratched up more than you would like, it is a very simple matter to mask off the hull at the waterline and respray the bottom.
Be aware that painting will add a few pounds to the boat.