The marine industry makes a cleaner for just about every type of dirt, grime, grease, and stain—including those nasty, tea-colored waterline stains. We tested seven cleaners; from left to right: 3M Heavy Duty Hull and Deck Cleaner (top), Davis FSR (bottom), West Marine Hull Cleaner. Aurora Waterline Stain Remover (top), Y-10 (bottom), Marykate On & Off, and Toon-brite.

Of all the stains a fiberglass hull will be subjected to in its working life—rust, oil, blood, and diesel exhaust, none, at least in our experience can be as tough to remove as a boat-long streak adorning the waterline. Sometimes this is the only blemish on an otherwise sparkling hull, and one that only an owner would notice. Other times, these streaks manifest themselves as ugly, tea-colored stains that make the boat look 10 years beyond its actual age.

While these stains can result from a variety of factors—algae, petrochemicals and, in fresh or brackish water, tannin—most are the result of metallic salts caused by the interaction of dissolved metals in…