If you are sailing the boat in to the US from Canada you have to at least check in to Customs. I don't believe you have to prove ownership
or anything if I remember what happened when I did it some years ago, but I would have the bill of sale
papers. If the boat was previously on the Canadian Boat Registry then it has to be formally delisted from that by the previous owner.
So first check the boat in to US waters and you have to do that at a designated entry point for wherever you are. Sometimes that means going out of your way to make that stop. Getting different advice is not good but check with local sailors or marinas
near by and they will tell you. There may be a way to just go to your marina and call it in but I don't think they do that much anymore and probably would not be happy. They may or may not come down to inspect the boat. You'll need your passport too (you and crew).
Then you can "import" the boat. I don't know if that is form 7501 or whatever. But it was easy when I did it. And then I had to register it with WA state (and pay sales/user tax - ouch). If the boat was manufactured in Canada then you will not have to pay an import
US duty. But you will need to be able to show it was made in Canada. If it was made in Europe
then you may have to pay an import
duty and probably some other paperwork.
You can get the boat "documented" with the USCG, or not, after all that.
This was all from a few years ago but I have not heard of any major changes since then. Good luck.