I had the PA State police pull up next to my boat because my sister was stiing on the gunwhale under way. Anyway, their balck fenders left tons of huge black marks all over my hull. Couldn't get them off with soap and water, but the MaryKate On and Off (or Off and On or whatever it's called) took it off like it was never there. I usually dilute it about 40% MaryKate and 60% water in a spray bottle. Don't breathe the mist or vapors it's nasty stuff, but it works wonders. Awesome for scum line at the waterline too.
