Buying from the US is much easier than you would think. Make a trip out of it and stay for 48 hours and you won't have to pay duties on $800 of the value. You don't pay any tax after paying duties. There's no special import/export forms for watercraft if they don't have a trailer. It's a very simple process that can save you a lot of money if the cost vs exchange rate work out. Ask the seller to make a receipt for a lower price and you'll be even better off. A ski that has a receipt for $3k should be $5900 CAD all-in with today's exchange rate. Expand your search down south.
Here's the duty calculator. Select sports, then jetski. Subtract $800 from the total value if you're going to be down there for a couple days, $200 if you're 24-48 hours. Whatever price the seller writes on the receipt is what you'll be paying. I like adding in as-is, to explain the low price. You can call the border beforehand to be 100% sure on what you'll pay.
Canada Border Services Agency Duty and Taxes Estimator
www.cbsa-asfc.gc.ca
What part of Ontario are you in and how far are you willing to go? There are often good skis in Quebec too. The SO*ON Freeride group on facebook has a lot of good stuff from people who know the sport.