We used Star brite Hull Cleaner yesterday and were pleased with the results. As the photo sequence shows, we sprayed it on a scum line that had built up over a season. The gel formula stayed put ( mostly) on the vertical hullside. Then we watched it dissolve the brown scum in seconds. Then we rinsed away the residue. No scrubbing or elbow grease was required.
Bonus Wear eye protection when using hull cleaners. And also wear gloves--we didn't in the above picture and our hands stung from overspray ( and we knew better!)