A few days ago the piling at the slip jumped right out in front of me while I was trying to dock and left some scuff marks on the top sides. I used a magic eraser to clean them after hosing the boat down.
Since I was sitting right next to the toe rail I thought I would see what the magic eraser did for the old dirty wood.
I was very surprised. With very little effort the wood changed from dark dark brown to a smooth natural wood color with nice grain. I think the "sponge" properties of the magic eraser really help soak up the oil and whatever else is in the wood while the abrasive take off some of the surface.
To be fair I never tried cleaning the wood with anything else -- but I can't imagine it being any easier.
The center of the photos looked like the black stuff on the right and left just a minute before the photo was taken.