any acid based cleanser will do for...
... organic stains like algae and mildew. I prefer liquid toilet bowl cleaner, but Tilex or Slimey Grimy will work also. What worked well for me when I was trailoring was dilute some commercial toilet bowl cleaner ($3.00/quart at Smart & Final) with water 50/50 into a hand-held sprayer and simply squirt it right on the surface then hose off with fresh water. No hard scrubbing please, you don't want scratch that gel-coat. Use a sponge or rag if necessary, but nothing abrasive.For petroleum based stains, like those you get from rubbing the dock, try acetone on a rag, then rinse with fresh water.I want to emphasize, do not hard scrub with anything abrasive or any abrasive cleansers. This is gel-coat, not porcelin.