I recommend Flood's Penetrol
, a low viscosity penetrating oil. It works like Armor-all on a car. Your deck needs to be clean first, and make sure its going to have a while to dry and don't put it on too thick...very thin layer with a rag, wipe off surplus after 2-5 minutes, before it gets tacky...and your boat will shine until you have to do it again. If it's too thick it will run after it dries and looks like spilled varnish and can attract dirt.