Hi Sonoma, here's the deal, I suppose there is two ways to do it, the legitimate way or the not so legitimate way. However, there is a snag with the not so legitimate way. I'll explain first the appropriate way (which I did)
Purchase the boat in the United States, ensure you have a seperate bill of sale for the boat and the trailer (meaning 2 actual bills) with the price clearly indicated.
At the border you declare the purchase of the boat and pay the necessary taxes and administration fee's. In this case you don't have to pay the duty due to the "free trade agreement" however, you do have to pay both the GST and PST at the point of entry back into Canada based upon the Bill of Sale for both the boat and trailer. As you may know, if you buy a resale boat (privately) in Canada you only have to pay the 1 tax a little after you transfer the ownership along with the one tax when you license the trailer. The other cost is a $182.00 admin fee to have the trailer registered in Canada since the trailer is considered a vehicle. Shortly after returning back home the government sends you a form which you take to an appropriate garage to have the trailer go through a safety check (basically it's just the lights they are looking for). Once you have the saftey certificate then you can license the trailer. To register the boat you simply need the Bill of sale and the original ownership. There is a web site
www.riv.ca which explains the whole process. In summary then, here are the costs and the things you must have
Pay both taxes at Border for boat and trailer
Pay the RIV fee of 182.00 for the trailer safety
Must have seperate Bills for boat and trailer
Must be able to clearly read the serial number on the trailer.
Must have ownership for both boat and trailer
The not so legitimate way would be to take your own trailer plate down with you and put on the trailer and cross back across the border as if you were in the United States on a trip or something and not declare anything at the border. You may get across no problem, however, the snag comes into play when you register the trailer because you won't have the safety form to allow the ownership to change and the plate transfer etc. I suppose you could live dangerousily and not register the trailer but that could lead to problems in the event of an accident or a routine check.
So, it boils down to how lucky you feel, the additional costs over living dangerousily is the extra tax and the riv fee of $182.00 which is not really that much more if you can get the boat at a reasonable cost. If you buy the boat here in Canada, you still have to pay the 1 tax on the boat and trailer anyhow.
For the other's, sorry to babble on but it can turn into a nightmare at the border if you haven't done your homework.
Sonoma, check out the website I mentioned above, its pretty well straight forward.
Bruce