also known as Wood Bleach? We had a dreadful ICW brown beard that nothing was taking off, including Soft Scrub with Bleach. Since I figured that the stains were tannin, Oxalic Acid is the first choice. We dissolved some of the crystals in hot water, painted on with a small cheap brush, and watched the stains disappear almost instantly. Also good for rust stains on fiberglass.

Oxalic Acid is a dry crystal, so it's easy to carry and store on your boat. It will not burn you, but as with hydrofluoric acid, its damage is internal through the skin, so caution and rubber gloves are advised.