I use the following cleaners in order of preference and performance.
First: Spray Nine - it can either be the "Marine" version or the landlubber version. No difference that I can tell.
Second: LA Awesome Orange Citrus or the All Purpose Cleaner - at the Dollar Store - I use it straight, no dilution
Third: Amazon Inflatable Boat Cleaner - most expensive and doesn't work any better than the two products above IMHO
I often use all three cleaners- each of them seems to work on some stains a bit differently.
I use a white or blue scrubby Doby-type pad on a handle to work the cleaners in a four square foot area at a time. Lots of elbow grease, but the results are apparent. I get these pads at Wal-Mart, the local Chinese emporium for about a buck each. Rinse before anything gets a chance to dry.
After all is clean, I spray on a couple coats of 303 Aerospace Protectant on the dinghy. I think that this protectant helps for avoiding UV damage and for repelling dirt. It is NOT the 303 product that is for canvas.
See my results in the pix below.
DO NOT USE Acetone or any cleaner that has silicone in it. The acetone will break-down the surface of the dinghy and damage it and silicone-based cleaners will cause problems if patches ever are needed.
BTW, this same cleaning procedure also works great on fenders and power cords.