The process is fairly straight forward. You pay PST and GST on the boat when you cross the border. If the boat was manufactured outside of North America you will also owe duty. If there is a trailer you also pay GST on the trailer and PST when you register it. You need a title and bill of sale for both. All you need to do is tell the Canadian border agent you are importing and tell you to pull over and go inside and complete the process. If coming by water you need to stop at a port of entry. If you have a trailer you also need recall clearance letter from the manufacturer. No need to stop at US Customs, that is for cars only.
You'll also need all the passport info for everyone on board when you check in by phone, which you must do if coming on by water. You will have the option of checking in at various locations depending on where you will arrive.
Whether you get inspected upon arrival is hit-and-miss.. in approximately 7 importations I've been involved in over the last 5 years we've been inspected 3 times.. the rest were all done over the phone. Also you won't be paying the inpection team.. you'll have to go to a customs office within 3 days.
the only other thing I'd add is don't succumb to the tempation to lowball your price (with or without the collusion of the seller) to save a few bucks on taxes. The Border services are pretty savvy these days with market values for any model/year of boat (they can google too!!) so if you really got a great deal make sure you have the paper to back it up.
How are you transporting the boat? There's a few things you need to think about there; for example if it's by land you'll need to properly prepare the boat for transport - check height restrictions, trailer ratings, vehicle hookups, etc. If by sea you need to make sure you have the correct safety gear on board because the rules and standards are different in Canada.
You also have to ensure you are buying a boat with a clear title (assuming this is a used boat?) - this process is not always straightforward and a good third party broker can be a big help here.
I've actually written an ebook on the process (contact me if you'd like info). I'm not looking to make a bunch of money, but since I know a lot about importing and have been involved in plenty of boat importations I decided to gather all this information and put it together into a single package.