Yes, that's where I bought my boat last summer. I have had some experience with this, so I went prepared. There is a U.S. Customs form for the repatriation of American goods. I don't have the form number right now, but I can probably find it or you might be able to Google it. The C Dory is built in the U.S., so there should be no duty on the hull. My trailer (EZ Loader) was also built in the U.S. I had both business addresses on the form and had the seller itemize the values on the Bill of Sale (because the duty rates differ if I had to pay).
I came back on the Black Ball (a private ferry) into Anacortes, WA rather than cross at the Interstate-5 U.S. Customs in Blaine. That way, I was already on U.S. soil when I went through customs. I had heard stories of people having to leave the new boat in Canada while U.S. Customs thought about what to do. I also had friends that spent several days at the Customs dock in the San Juans when motoring their new boat back. They finally hired a customs broker and paid for him to fly up from Seattle. Both of these situations seemed to be geared towards getting the new owner to quickly agree to whatever duties Customs levied just to get over the boarder. Reminds me of Mexican security waypoints.
I got off the ferry, showed Customs my form, and was told that I had the wrong form and would have to pull over and wait. One issue was my Yamaha motor. I told them I didn't know where that was made (and we all grinned). But I was aware of a policy that if the imported goods are more that 50% U.S. made it is possible to get a duty waiver, in part so that there are no protracted computations over what percentage of the total value is foreign made. When they asked me what the motor was worth, I told them that I hadn't yet started it (true) and that would make a huge difference in the value.
They went off to confer with whom ever they confer with. After half an hour, they printed the "correct form" (which was the same as my form but computer generated so that I'm now "in the system") and then stamped it with a bunch of official stamps. Again reminded me of Mexico.
Off I went with a full wallet. Your experience may vary, but I hope not.
Mark