This is an interesting topic. For thirty years or so I have not registered my dinghy and motor anywhere, having lived in RI/ME/RI again. Many years ago I was told that if you have a CG documented vessel and use the dink only for shore transport, you didn't need to register the dink. My dink is labeled T/T Maxine and carries nameboards with my CG doc # -1. In all those years I was hassled only once on this, actually in Menemsha Pond by the harbormaster who didn't want me to be anchored where I was. But he backed down when I showed him the numbers, etc. I believe this is typically what happens as the states have their own view on what the legalities are, and they want their money. So when I read your post Tom, I went googling and this is what I came up with:

"Where the Coast Guard issues numbers, the following classes of vessels are exempt, under Section 12303 of Title 46, United States Code, from the numbering provisions of Sections 12301 and 12302 of Title 46, United States Code, and this part:
(a) A vessel that is used exclusively for racing.
(b) A vessel equipped with propulsion machinery of less than 10 horsepower that:
(1) Is owned by the owner of a vessel for which a valid certificate of number has been issued;
(2) Displays the number of that numbered vessel followed by the suffix “1” in the manner prescribed in § 173.27; and
(3) Is used as a tender for direct transportation between that vessel and the shore and for no other purpose."

Here's the reference:
http://www.law.cornell.edu/cfr/text/33/173/13

Sooooo, what I'm going to do is have a copy of this reference aboard in case I am ever stopped again.
Thanks for bringing this up Tom.