: oops sorry, over 13 has to be registered

That looks suspiciously like a law or regulation made by someone with no knowledge at all of the things the law was to be applied to.

I can think of a few reasons why it might be a good idea to have a registration requirement for something as big, heavy, fast and potentially dangerous as a 15' powerboat or sailboat, but I can't think of any reason why it should be necessary to register something that is unlikely to weigh more than 30 or 40 lbs and is most unlikely to travel faster than 5 or 6 knots.

Don't the legislators understand the differences between powerboats and kayaks?

Or is there something I don't know about America? :-)