My first brilliant solution to the boat numbering problem was of course using the self-stick numbers from Home Depot. These did stick to the inflatable PVC hull, but inspired no confidence as the corners peeled off immediately.
After some research, I figured that I need to paint the numbers on. I ordered stencils, made a template with stencils, tape and old newspapers, and sprayed white Krylon Fusion onto the boat. This is a surprisingly time-consuming process - making the template to look straight in the first place can take a while, then masking and taping, and more masking.... and in the end, you still wind up with overspray everywhere because the letters weren't absolutely flush with the boat. Now you need to use acetone to clean up the cloudy edges, which also takes forever and goes through paper towels very quickly.
The real surprise comes in a few days - when you realize that the paint isn't drying. It's still sticky - scotch tape sticky. So I called Krylon, and they confirmed that the paint will only work on rigid PVC. Thanks, Krylon, all of my spray paint works on rigid PVC.
At this point there is nothing else left to try but that which you should have spent your money on in the first place - boatnumberplate.com .... live and learn.
Update: The paint remained sticky for about a month, and during this time deflating and rolling up the boat would lead to paint transfer. Surprisingly, as time went on the paint became less sticky, and I may not bother to use the boatnumberplate patch since that's rather permanent.... but this paint I can remove with a bit of acetone.