A hull manufacturer can't assign hull numbers to a hull as per USCG regulations. If I sell the hull only, I can't put my numbers in the hull. A registered boat builder that builds the complete airboat must install his assigned HIN in the hull. That puts the responsibility for safety on the builder. Builders that are registered with the USCG are inspected every so often to insure the safety requirements of the USCG are adhered to. Registered Manufacturers also have to furnish a Statement of Origin with the invoice.
An Airboat also requires an Exemption to be legal. Very few has this exemption. This exemption because airboats don't meet the USCG requirements for Floatation, Capacity, Fuel System, Electrical System, and I will have to look at my exemption certificate to see what the last one is. I will post a copy of it.
This may be why you can put your own boat together for less than a Boat Manufacturer. This is also why I value a used Manufactured airboat for more than a Homemade airboat. The State of Florida is a Safety failure in the issue of Homemade Titles because no safety inspection is made. If the homemade boat comes apart and kills someone, then the State should be charged with the deaf of the person. That being said, I know some homemade builders that are very good at building a safe and quality airboat.

You can do a search on this site about USCG Regulations or Hull Manufacturer.
Thanks, Chuck