l.p. is best or a 1 part poly urethane. after prepping put on your first paint coat, while the paint is still wet, use a can of silica sand for non skid. poke holes into the can of silica, so it's like a salt shaker, and cover the wet paint with sand. let it dry, and vacuum the excess. then put a couple coats of paint over the silica.
Proline 2 part LP Paint - Cheap and high quality
you also have the option of ground tennis balls or for the sand look with out the headache if you have to grind it off to repaint Awlgrip makes fine - medium and course non skid particles which would be applied like Mattanza suggested
Is it textured gelcoat now? If you want to do it right and never look back, you sand the old surface to 120 grit smooth, prime with 545 awlgrip primer, then awlgrip topcoat over silica texture grit. It sounds harder than it really is, but that floor will withstand all stains and wear forever. 1 part paints will get checked up in time depending on how much traffic is on them.
+2 on Awlgrip. If you go with the primer 545, and spray Awlgrip topcoat, it wont cover your up factory non skid. I replaced the deck in my Skippy so I had to add a non skid and used a rubber particle for non skid applications. The rest of the boat's factory non skid,, gunnels and bow area,, is " Spray" painted with Awlgrip, and the non skid is fine. If you roll it on, you may fill the factory non skid, IDK. The paint is worth every penny, just follow the directions and you be fine,,,gregg