I just used this hull cleaner from a marine shop last night on mine. it was super easy to do and took all the stains right off.
Get a brush for washing cars, put a little of the hull cleaner in plastic tub, dip the end of the brush in and rub it on the boat. it literally removes the stains in about 10 seconds. Then just rise it off with water. Took me about 20 minutes to do the whole boat.
You might want to wax it afterwards though, that will help slow down the staining process next time.