If you're buying a used boat from a private party in Canada, you still might owe taxes when you cross back to the US. Here are the rules:
Limits for U.S. Residents
If
you're a U.S. resident visiting Canada for less than 48 hours, you can
bring back $200 worth of goods duty-free. For stays longer than 48
hours, the duty-free limit rises to $800. This $800 limit can only be
used once every 30 days. If you return for another shopping trip in less
than 30 days and have already purchased $800 worth of goods, your limit
drops down to $200. People who live in the same house can combine their
duty-free limits, but they must travel together.
However, the market for used WW boats is absolute crap these days, and with the
Canadian dollar worth 75 cents US, you can get some really great deals on a used
boat up north.
So... if you buy a used boat from a private party on a day trip to Canada, you're probably safe reporting a sale price a little higher than the limit, maybe $250 US or so. If you spent more on the boat, you've got a few options: 1) tell the truth and pay the tax. 2) report a sale price of $250 or so, or 3) stay up north for 48 hours and the report the true sales price at the border (up to $800.)
The thing about Option 2 is that there are so many cheap-as-dirt plastic rec boats around these days, unless the border agent is an experienced kayaker, he/she is not going to be able to tell the difference between a quality WW boat and a POS K-Mart Special. So it's entirely plausible if you choose to report a sales price around $250 or so. Especially if the boat has the usual scratches.
Anyhow, you have lots of options! Choose the one that works for you.