Go to Wal Mart or Dollar Store & get a bottle of stuff called "THE WORKS", Tub & Shower Cleaner. Instantly removes the yellowish / brownish stain. $1.25 or so for a 16 oz bottle. A friend in Gallatin County Bass Club put me onto that stuff 10 years ago. I've since spread the word and have been thanked by many. I just wet a towel with it and wipe it on. POOF the stain is gone and your boat's new again.