Marykate's On & Off, available at Amazon, 32-ounce bottle for $12.47
If you keep your boat in the water for the season, or year round, you'll notice some ugly brown stains along the waterline. You'll also notice that a simple scrub brush and soap combo just won't cut it.
On & Off will remove any waterline stains and exhaust stains, as well as stains left from barnacles and algae, and leave your hull looking absolutely cherry.
It's just recommended, and probably required in some places, that you don't treat your boat with this stuff in the water. It's highly volatile, and you don't want to kill all the poor little critters clinging to and dwelling around your dock or mooring slip.
I use this stuff in the beginning of the season, before I paint the bottom and dunk the boat in the water. That way, you can clean down to your bottom paint line and paint over what paint it strips along the edge. Wash your hull first, then wipe it dry, and then wipe on the On & Off with a rag (and gloves — be sure to use gloves). Let it sit for a few minutes, remove with another rag, and rinse and repeat as needed.