If the boat is made of PVC, you can buy paint for PVC and paint over the area, then paint your numbers on over that. If it's Hypalon, I can't offer any advice other than to do a web search on painting Hypalon.
I had a PVC dinghy and I painted the numbers on just fine. I tried it on a Hypalon boat and the paint pulled off with the stencil. Every bit of it.
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