I think the USCG supplies to towns the names of all documented boat owners who list their address in that town. The town then makes sure it collects the excise tax from all boats in its jurisdiction. I pay excise tax in Harpswell, where I live. But my mailing address is Brunswick (I use the UPS store) and I used to routinely get a letter from Brunswick asking me to pay the excise tax. Every year I had to go in and prove to them I live in and paid the tax in Harpswell. Brunswick could only have gotten the info from the USCG database where I use my Brunswick mailing address on the documentation.

When the boat is overseas I don't pay the tax. The annual excise tax is around $120 for my Nordic 40. Pays the salary of the tax collectors, I suppose.

Edit: even when I register the mooring I own in Rockland, I have to show that I have paid the excise tax. Seems like coastal towns are pretty diligent making sure it gets paid.