Don't use Zinser - its a special undercoat for marked or fire damaged walls. It might work, but its too expensive for no real benefit to you.
Like I said, good quality sealer undercoat for wet areas like bathrooms seems to work well.
I know its a pain for cleaning, but non water based paints are far superior to any water based if you are going to walk on it. If you want to save your brushes, after you give them a good rinse out with Turps, leave them to soak in a Washing Machine Powder/Water mix ( about the same as you would use for clothes) in a container for a few days, and they will return soft and ready for re-use after a good final rinse
Do NOT use crushed rock or salt in the top coat. You wouldn't put 80 grit sandpaper down on a floor in case you fell over and gave yourself gravel rash, so don't create your own 80 grit paper on the deck.
I just tried to ring a guy who knows, but he used some sort of plastic particles especially designed for deck grip.
Edit : Peter used a special decking compound that you trowel on, and gives you a stippled, rubbery finish.
His friend on another boat used crumbled cork, but that still leaves little hard paint spots when they get knocked off. Its better than sand and other hard stuff.
Another great hint, when you apply the top textured layer, do it in a "hopscotch" pattern, and mask areas of with tape in alternating squares. Then you have somewhere to stand without getting stuck as you apply the final, textured coat. It means you have to do it in two stages to allow the first lot to dry, but it will be a better result.
