I bought a new boat from a Canadian delaer 10 years ago.
A few hiccups, but it went well. We bought FOB my New York address. Dealer delivered boat. Had boat US Cast Guard documented as required as part of the financing. US CC would not accept Canadian notary...so dealer traveled to USA to have that done.
Used boat from a non-delaer may put more resposibility for paperwork...on you. Run this past customs...they can set you straight on duty... Go slow...there may be traps to avaoid...don't assume SOP in the USA applies.