If dead leaves have stained you boat, it means that your gelcoat is now porous (for lack of wax, etc.) and the tannins from the leaves has entered the porosity of the gelcoat.
Best remedy to remove tannin staining (on deck, etc. as well as the tannin waterline 'mustache') is to use any cleaner with Oxalic Acid. Will INSTANTLY remove the stains. Oxalic is found in many cleaners or cleansers (dont rub a cleanser on gelcoat - just apply and let soak) - BarKeepersFriend, Zud, etc. etc. Oxalic Acid (in crystal form) is sold in most paint & hardware stores as "wood bleach", etc.
Last edited by RichH; 10-29-2009 at 12:18 PM.