If you pressure washed it and still have black and yuck the question is "does the yuck transfer?" (i.e., if you sit on an area that's yuck or rub against it - by accident or on purpose - do you get any on you?). If no, take the boat down any river w/suspendend sediment (Colorado, Green, most any south of Idaho or PNW until at least June/July) and call it good. If yes, same approach may work but I'd buy Soft Scrub at local grocery and judiciously apply to a non-critical area (i.e., where no butt meets the rubber) - if it cleans up and no longer rubs off, you're good to go.
Regardless, once you get the worst off, thoroughly dry and apply 303.