Walmart!.
There is a hull cleaner in the boat section called "Brites" hull cleaner. I was really surprised how well it cleans the scum and discoloration off my hull. It's rough on your hands, so you may want to use gloves. As long as you got manly hands with no cuts, you should be fine..
Let it sit for the two minutes it suggests, then wash it off with a water hose. I was amazed the first time I used it...........:cheers: