Ryan is right. Try StarBrite Hull Cleaner. It is a mild acidic cleaner. I'm not sure what "high rock ring" is, but this cleaner works great getting off lake scum, aka dirty water, stains, etc. I've used it several times on my Supreme and it works great. Apply it with a damp sponge and rinse off within a couple of minutes. If it's real bad, you may have to reapply. I've always washed the boat after applying, just to make sure no acidic residue is left behind. Also, I'm fairly certain it will strip any wax job off the boat; that's the big downside. You can buy it at West Marine, Overton's etc.
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