Coast Guard and documentation.  The  CG offers a national registry for vessels over 25 feet.  This is at it's basic level just a registration and tag service.  Kinda like your car.  What's different on your car is that you pay taxes so that the state will issue the tag and registration.  With the CG you pay a "document fee" to get a registration and you make up your own tag (the name and hailing port of your boat).