Just went through that process myself. Bought a 19' North River from southern Washington. BTW, there's no sales tax to Canadians buying in Washington or Oregon.
Here's a little more to add to what's already presented.
Payment - if you are certain about the boat you are buying, and want a bit of wiggle room to negotiate price, you can get a Bank Draft or Money Order made out to the seller with an amount that's lower than you will eventually pay. Then take a few thousand U.S. Cash with you to cover the negotiating difference. If you're going to look at a couple, as I did, you're better off taking all cash. I could not find any 'bank' in Washington that would cash a money order made out to ME unless I had an account at their institution.
Declare the cash at CBSA as well us US-CBP on your way down. It may take you an extra 1/2 hour stopping at the Canadian side, but makes the US interview process a lot smoother. I had $25k with me and US side didn't even count it partly because I had already done the Canadian. You can print the Canadian declaration from the internet. I filled out 3 copies in advance. If you ask for help - the staff at both border security offices are pretty helpful.
When at CBSA on your way down, grab a couple of VEHICLE IMPORT FORM - FORM 1's. You can then fill in a lot of the obvious stuff before you're called into their office to pay the tax. IMPORTANT - if you declare that you're taking down $20,000 for a purchase, and only pay $16,000 for it, CBSA is going to be looking for you to be bring back $4,000 US cash (or receipts for other stuff you bought)
Title - US boats and trailers have an actual Certificate of Title document to prove ownership. The form also serves to pretty much prove there is nothing officially owing on those assets. When there's a loan in place, the lender usually keeps hold of the title documents. If the boat seller has the titles, and the registered name matches their personal ID, then you're pretty certain that there's no liens on them. The Seller will have to sign the Certificates 'over' to you. You will also NEED those when bringing the equipment across the border.
Border Payment - you sometimes only need to pay the GST on the trailer at the time you "import" it across the border. In my case they collected GST on both the trailer and boat.
All of the advice you've been getting can seem pretty intimidating, but if you make a list of what you need to take and do, then follow it step by step, it's pretty easy. When I look back, I was pretty stressed about not doing everything right; but when I look back it was a pretty straight forward process. None of the sellers I met with had any problem with me checking my lists and making sure we didn't miss anything.
Trailer Plates - if the trailer plates and insurance are current, you the buyer have a few days to tow the equipment back here with the US ones still attache. You will need to send them back to the seller.
Actual "Importing" - I was told at CBSA that if I ever want to take this boat back to the US, or sell it to someone there, it would be best to actually "import" it. Apparently that process requires a 3 day impound of the equipment. They however also commented that If I were just trailering it down for a short time the US-CBP would probably: not notice, or not care. I did not "import" mine and apparently the majority of small private boat purchases do not go through the Imported process.
Be organized - if necessary get a half dozen file folders, or big envelopes, for your trip. One for Craigslist / Boat Trader / iBoats... Ads, one for Canadian docs, one for US docs, one for boat Bill of Sale and Title, one for trailer Bill of Sale and Title, etc. That will help with the border guys impression of how serious you are, and also make finding papers easier when asked for them. KEEP ALL YOUR PAPERWORK !!!! receipts for gas, lunch,..... helps them see where you've been if you need to prove it. Probably the trickiest bit of the whole process was checking the actual serial number stamped on the boat hull, engine, drive unit, and trailer against the Titles and Bill of Sale
After you're back you'll need to do 3 important things:
- mail back the license plate
- go to an ICBC agent and register the trailer into your name (take ALL your paper)
- get an Application for Pleasure Craft License from the internet and get it off to Transport Canada form # 84-0172e (mine was back in about 20 days)
other:- if the boat has a kicker and/or downriggers on it, ask for a price reduction if the seller takes them off. (they're almost as cheap here and there's no exchange, etc.)