Boats are like cars in that they must be registered with a licensing authority. Depending upon the size and use of a boat, some vessels are required to be documented with the U.S. Coast Guard National Documentation Center. This is only required if the vessel is more than 5 net tons and engages in fishing or coastwise trade. In recreational boating cases however, the Coast Guard only requires "vessels equipped with propulsion machinery to be registered in the state of principal use."
To comply with the Coast Guard regulations, you must register your boat with your state's licensing division. Much like when you register a car, you will receive a certificate of number. The numbers must be displayed on your boat in a specific manner, and the owner/operator must carry the valid certificate of number (like a car's registration paper) whenever the vessel is in use. The best way to ensure the certificate number is readily available in the event of a law enforcement boarding it to put in a zippered plastic bag and store it in a secure, watertight compartment aboard your boat.
Check with your state for specific licensing requirements. Click next to learn how to place the numbers on your boat.