Different cleaners attack different kinds of crud. You may need to use a few different things, one after the other. Simple Green has received a mixed review here. It has worked well for me in many situations, some of which were fairly extreme, like rehabilitating 1920's vintage machinery. I consider that to be a very good place to start. A de-greasing cleaner like Purple Power Cleaner from the auto parts store, on a rag, may be a good next step. A scouring powder, like Bar Keeper's Friend, or Ajax or Comet, may be a good step #3. If you make a paste out of that stuff & water, then let it sit, it seems to pull some grime out from under the surface sometimes. After that, I might try a touch of bleach, perhaps diluted about 3:1 or more. Normally, the longer it sits, the better it works. If that doesn't do it, then the pressure washer may be good last resort, but be careful with that thing. When I worked in a boat yard, we used to cut wooden 2 x 4s in half with the one we had. Be sure to keep the blasting tip far enough back from the surface that you are trying to clean, so that you don't damage the fiberglass. Also, be prepared to clean up the mess that it will leave behind.
One option for defeating the bilge pump may be to disconnect the hose from the thru-hull fitting & stick it into a big bucket temporarily or connect it to a longer hose that can reach a suitable drain.
