In any case, you will need a bill of sale for the trailer in order to register/plate it.
The last boat I had sold, I tacked on the boat information in the "additional comments" section of the bill of sale form. A separate form for the boat is a good idea.
I also provided a copy of my boat registration. Makes it easier to transfer registration when you have it all written down for you.
That being said, make sure the registration papers match the Hull serial number (usually on the plate that is riveted to the hull, on or near the plate that lists the capacity etc. and the registration number (the stickers on both sides of the hull), manufacturer etc.