Dye mark make a product called Tread Rite, I think it's available from blackwoods, haven't used it on a board but at work on slippery wet concrete it worked a treat!
Thanks von. I saw the dye mark stuff was recommended on another thread. Bought a can from bunnings for $10 (so not a big investment) and can says not suitable for salt water immersion. Will have to look into it, ta