Also in a diluted form as "Barkeepers Friend" but far cheaper bought as crystals and diluted yourself.

It'll clean rust stains off stainless, the brown "Waterway Frown" off the hull of boats, etc. Used it in my furniture repair and refinishing business for years on stains on furniture before refinishing.

I even use it on my painted hull. Dilute with water (salt or fresh) wash down the stain, let it sit a bit and rinse. Using it on the waterway frown simply wipes it off, like erasing the stuff.

DON'T get it on the Hypalon of a dinghy- it'll bleach the color

I keep a container aboard always. Another out in the shop.