paper work for importing a boat:
-Bill of Sale for boat and trailer defining the price for each.(or separate "bill of sale" for each)
-title for Trailer.
-if the trailer is 12years or newer "recall declaration letter" from the manufacture is needed.
the boarder doesn't need the title for boat, but you will need it to sell the boat later.
you will pay at the border:
both taxes on the boat, and GST only on the trailer, for the value in Canadian dollars at the time of import.
PST on the trailer will be paid when you register the trailer with icbc.
Caution:
if the bill of sale looks to be cheaper then the value of the boat, the border people will make life miserable during the process, a copy of the "for sale add" will help in your favour.
Bonus but not needed:
On high ticket items, all the above paper work should be sent to the US border services 72hours prior to exporting from the US. They will verify all paper work, check for liens, and allow the export.