If you are buying private you do not pay tax. Once tax man gets wind that you have registered a boat, they will inquire as to how much you paid, and will come after you for the tax. If you do not have a receipt, they will come and inspect the boat, and charge you whatever they think it's worth (basically).
So as mentioned before, keep your receipts.
Make sure you get the ownership for the trailer, and a bill of sale so you can get it put in your name.
The Bill of sale should include details about the boat, motor and trailer. (boat make + model, motor make + model, trailer make+model, etc..)
If the bill just says "bought a boat, motor and trailer for $xx" then people at the DMV might throw it out. That's what they tried to do to me. They need specidics acording the woman I talked talked to.
As far as the other details, I can't really help. I havn't bought a boat worth more then $1000...