send an inquiry to the 'boating law' people. They might give you the answer pro bono.
I did send an inquiry, and Todd Lochner responded by providing some useful insights both privately and publicly here. I know that he is being careful not to violate the forum's self-promotion rule, but I do want to publicly thank him for his contributions here. His website has been linked several places, and is worthy of one additional link because of the wealth of useful legal information that it provides. However do realize that laws change over time, so his information should be used for general background and not used as legal advice for any specific case:
Boating Law - Boat Tax - 2008
This above page provides a general update as of 2008. There is a pull-down list that shows a couple dozen other topics of interest, including the offshoring option that has been mentioned in this thread.