Just go to the auto parts store.
A couple years ago a friend's kid got heavy railroad grease all over his clothes while hiking down the Royal Gorge. I gave her a cup or two of "Fast Orange" with pumice,--- the stuff that you clean your hands with after working on the car.
She pre-treated the clotheswith the stuff, scrubed them and they came out good as new in the wash. If it works on cloth, it should work on a boat easy.
"So in two seconds, away we went, a sliding down the river, and it did seem so good to be free again and all by ourselves on the big river and nobody to bother us." -Mark Twain (The Adventures of Huckleberry Finn)