The marine industry makes a cleaner for just about every type of dirt, grime, grease, and stain—including those nasty tea-colored stains on the waterline. We tested seven cleaners, which are pictured on the wheel well of our trailer, with a stain-free hull (thanks to the winning cleaner) in the background.

If all the stains a fiberglass hull can sustain—rust, oil and blood, for instance—none, at least in our experience, can be as tough to remove as a boat-length streak along the waterline.

Sometimes it’s the only blemish on an otherwise sparkling hull (like the publisher’s 28-foot Boston Whaler), one that only the owner would notice. Other times, it’s an ugly tea-colored stain that makes the boat look 10 years older (PBR’s Twin Vee Awesome 22 catamaran prior to this test).

While these stains can be the result of 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…