I have never needed or desired to "document" a vessel. I understand that for the pain of dealing with the US Federal Government and paying a fee to renew every five years, it makes some international vessel sales and financing a little easier. You do not avoid paying whatever state fees are due for keeping your boat in the state, and most states document this with that little tax sticker annually. You do not post numbers on the boat, but must register her name and give a hailing port, put that on the outside, and permanently stencil the federal registration number on the inside of the vessel. My view is that I would do that if someone held a gun on me, otherwise it's too much of a pain.