It is a relatively easy process. If the boat was manufactured in North America (covered by Nafta) no duty is owed. You have to pay PST.
You have to fax documentation
on title for boat and trailer to US customs
72 hours in advance. Fax number are listed for each of the crossings. They complete a title search and assure that the boat wasn't stolen and stamp the titles.
Now stop at Canadian customers. You need to have certificate of non-recall for trailer. Titles and bill of sale
for trailer and boat. You pay the PST and now they are Canadian. The boat should be registered with Service Canada
and the trailer must be inspected by Canadian Tire to assure it meets Canadian standards. You have 45 days to complete the inspection
Check out the customs
Canada site for vehicle importing to get detail on the process.
You can also use a broker to do the import