A couple years ago I tried about 15 different products.... and of those, Spray Nine worked best. Can't remember if it was the "marine" version or not. It was NOT effortless; stiff brush and lots of elbow grease still required. That test included Soft Scrub, not great. Didn't include GoJo. Included Simply Awesome, or some such. Underwhelming. Included Mr. Clean erasor pads, not bad but even more labor intensive.
The last year I tried a product called Marine Strip, said to be OK for vinyl. Worked better, still not effortless, needs some soak time and a brush... and then about 6 months later, the vinyl was a little sticky.
Since then, I found some decent covers made of Sunbrella, from Harken. Those seem to be OK; I think they'll pick up splinters and dirt along the way, but I'm guessing maybe a periodic dismount/wash/remount might work. Previously, I'd tried some acrylic covers; they disintegrated relatively quickly. The cover for our Polyform buoy is acrylic, so I don't expect long life from that.
-Chris