I've had good success with Interdeck, a nonskid version of Brightsides. Three seasons now on the cockpit floor - started kind of bright white but has now faded/discoloured to a remarkable match with the gel coat finish elsewhere.
Great non skid properties wet or dry, smooth non abrasive finish, relatively cheap and easy to apply.
But I've also heard good things about Kiwigrip..
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)