I also use spar varnish but add aluminum oxide powder to the parts I want grippy. It's worked great for me. Previously I've used deckover type paints, varnish with sand and some varnish with rubber in a driftboat. The rubber doesn't last well as the little spheres deform and pop out after a time. I've been very happy with the aluminum oxide and it doesn't change the appearance - you can still see the wood beneath. It's also very fine grained and doesn't abrade body parts. The perfect amount of grit in my opinion - it's kind of like 220 grit sandpaper, but with much fewer grains.
"If you dont do it this year, you will be one year older when you do"