Just bought my boat in Canada. However, this boat was originally a US boat, so all duties were paid once. So I did not have to pay duties.
As for sales tax, that's up to the state you intend to register it in. Some want sales tax and some have a personal property tax. Doesn't matter if it is documented or not.
You should use a broker to bring the boat in. I used Norman-Jensen. Laurie Boucher @ 810-982-8183.