So.. I've done this twice now. The first time, in 2009? I imported my Canadian-built boat from Vancouver, BC to Washington state. I didn't use a customs broker. The biggest problem I had was getting my $US converted to $Can... Everybody wanted to take a couple of percent for the transaction, and I didn't want to pay that. The Canadian escrow company handled the currency exchange for me for a nominal fee. That was much appreciated.
The way it worked was that the paperwork was all handled by the buying and selling broker and the escrow company. When I arrived in the U.S., the customs folks came down and I filled out some paperwork, and paid the nominal fees for the transfer. (You need exact change).. the whole thing took about 30 minutes. .. Easy
That being said, I helped a friend bring his newly-purchased 1D35 from Vancouver BC to Washington state. He thinks that the escrow company did the currency exchange as well. He did use a customs broker, though. He was told by the selling broker that the law had changed and that a customs broker was required. I surfed around a bit to see if that was the case, and could find no evidence that was true. I didn't have much confidence in the (different) selling broker in this case. The experience with the customs officers on this boat was essentially the same.. Took about 30 minutes. Easy.
Since Windsor is so close (To those not familiar with local geography, Windsor is actually south of Detroit).. I'd be inclined to try it without a customs broker. If you get turned away, then you could always sail back to Windsor and try again... Getting kind of late in the year, though...
I know that my boat transfer officially took place at sea, although the previous owner wasn't present. This avoids Canadian taxes.
As others have indicated above, if the boat is Canadian or U.S., there is no duty, due to NAFTA.
I'd be happy to dig up any detailed information you'd like.
Oh.. and this better be a sailboat we're talking about here...