I live in Phoenix, AZ. I purchased my boat in Fort Meyers, FL. The dealer there did not charge me sales tax as I went to Florida to pick up the boat. When I got back to Arizona I had the boat documented and paid no sales tax.
This is how it works in AZ. All boats (motorized) in AZ need to be registered at the Game and Fish Department. They will want to see a bill of sale showing that tax is collected. After paying the fees a certificate of number (registration) is issued for your boat and you put on the appropriate numbers. If the bill of sale doesn't show any sales tax collected, the Game and Fish Department will not issue a certificate of number. They make you go to the AZ Dept of Revenue and pay the tax due.
How then do you not pay tax on a documented boat? In Arizona, the state won't allow dual registration. If the boat is documented, the Game and Fish Dept. won't allow you to register it. If you can't register it, no one knows if you paid sales tax or not. Easy.