Originally Posted by SiXeVeN
...Almost my entire topside is a molded in non-skid pattern...
A gentle lesson on terminology for you here... the "topsides" of a boat is the area between the waterline and the deck edge - ie the surfaces of the hull itself above the waterline. I doubt that that is in nonskid!
You are, of course, referring to the deck
Powerwashing and a good stiff brush generally works best, but it's always difficult to get the grime/mildew/grunge out of the deepest parts - which you'll need to do even if you plan to paint over it. You do need to be cautious that you don't fill the nonskid pattern and make it ineffective.
If you do paint, adding a grit (sand, walnut shells, or a more commercial product like "Grip-tex) will help. Getting a uniform appearance takes some practice.