So I got curious and found the On and Off MSDS. It's mostly Hydrochloric Acid (another name for Muriatic acid). but also has lesser amounts of phosphoric acid and Oxalic acid. All diluted 1:2 with water
So it's probably as safe for the dinghy
as Muriatic acid (and pretty damn dangerous to humans). But I've used it a lot on ICW
stains and have about the same number of body parts
(I do wear gloves and glasses, work outdoors, and use a pad on a short pole to keep my distance)
I really like the gel for ease of application.