David,
I assume the walnut stains are on the white topside gel coat. Try spraying it 
down first with a product made by "Mean Green" called Mold and Mildew stain 
destroyer. Let sit for five minutes and hose off. If that doesn't remove all of 
the stains, try scrubbing the persistent ones with soft scrub with bleach and 
a green scotch brite. The even more stubborn ones might require the use of a 
cleanser like Comet.
Mean Green products are available at Dollar General Stores.

Rummy