United States It is PSUBS opinion that recreational submersibles are considered "boats" by both the federal government and the Coast Guard.   If you experience issues registering a submersible in your state these legal references may help state authorities understand how they should proceed with registration of the vessel. U.S. Code, Title 46, Subtitle II, Part A, Chapter 21 - ( 46 U.S. Code 2101 ) 46 USC 2101(34)(a) - Defines a recreational vessel 46 USC 2101(49) - Defines a submersible vessel USCG NVIC 5-93 Chapter 1, B(4) - Defines recreational submersible and that they are regulated by rules for BOATS, 33 CFR Subchapter S.