Very interesting. First I'd heard of this. I'd always taken it as gospel that documentation relieved you of boat registration requirements.
I decided to check on the situation here in Maine since I just sent in my application for documentation today. It got even more interesting.
One of the lawyers here check me on this. It says in this document from the state site
http://www.state.me.us/ifw/atv_snowmobile_watercraft/registration.htm#watercraft
that vessels are exempt if they are REQUIRED to have federal documentation. Since it is optional for yachts, I wouldn't read that to mean just being documented provides exemption. I called IFW (the registering agency up here in the corner) and was told that I won't need to renew the state registration or even inform them after I receive my certificate of documentation. I do have to pay the town excise tax and get a sticker I've never seen or heard of.
However, then she said that all vessels over 5 GT are required to be documented so my question was moot. That's clearly not the case. I've been on state registered and numbered vessels up to 100 feet. The MD research vessel is one because she is foreign built.
Which makes me wonder. Does the state agency know what it's own law says? You can bet I'm not going to ask anywhere but here though.