In order to import the boat into Canada you need to provide a certificate of origin that states the country of manufacture of the boat which must be provided BY THE VENDOR, an invoice and bill of sale (the latter two could be one document - i.e.: an invoice marked "Paid in Full"). Also, although it is not mandatory, it would be a good idea to complete one of these forms, as you will be required to provide the information it asks for:
If the vendor can provide you with a copy of the title document, and a COMPLETED application to delist the boat from the US registry or transfer title (I am not sure of the exact process there), and if that title is in the same name as that of the vendor, then it would be good to have that. If not, then you should request a witnessed affidavit stating that the vendor has clear title to the vessel, and that title is in the process of being delisted and that title has passed to you.
Depending on the origin of the boat, it may or may not be NAFTA eligible. If it was made in the US, then it is. If it was made elsewhere it probably is not, but MAY be. This is the information that you need to know as far as rules of origin go - note that your boat would be classified as 'casual goods' rather than commercial goods (assuming you are not bringing it into Canda to resell it:
It would be a good idea to take as much documentation with you regarding the history of the purchase (advertisements, emails, etc.) as you can.
If possible, accompany the boat across the border. If you are trucking it, then either ride with it, or follow it in your own car. If you are sailing it, then you can report in to Customs when you arrive, and they will refer you to the appropriate office to make your formal declaration.
If the boat is NAFTA eligible and duty free, then you will only be required to pay GST on the purchase price. If it is dutiable, then you will be required to pay 9.5% duty on the purchase price, and the GST will be calculated on the sale price and the duty.
If you are taking cash or a certified cheque, or ANYTHING that could be considered negotiable into the US to pay for the boat, and if that is more than $10,000.00, you need to declare this to Canadian AND US customs when you are exiting Canada and entering the US. If not, it may be confiscated.
If you cannot accompany the boat, it may be worth using a customs broker to handle things for you.