If "On and Off" or toilet bowl cleaners or other acid product remove the stain, it's not tannins but something else. "Tannic acids" are neutralized and removed by basic products. Stains in stainless coffee carafes and teapots are typically removed by TSP or caustic soda. I suspect the brown stains on boats are iron oxides and are therefore most easily removed by oxalic or nitric acid.
Jim
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