As the fool said, registration and documentation are two different things. A state registration is usually the same as a motor vehicle registration and often required even for federally documented boats if there is an engine on them.
The USCG will answer phone calls and aid you in the Documentation process, which is typically painless and can be done at about 1/4 of the cost that a "documentation agent" will offer to help you with. But if you are financing the boat--the lender may insist on doing that in order to make sure it is done properly (for their title security) and soak you for that to the tune of $700-800.
There's a lot more paperwork to be done filing a "short form" tax return.