Caveat: I am a lawyer, but not your lawyer. Free advice being worth what you pay for it and all that . . .
Starting in 18 U.S.C. 921 (Definitions), I find this:
18 U.S.C.A. § 921
(27) The term “motor vehicle” has the meaning given such term in section 13102 of title 49, United States Code.
Following that thread over to Title 49, I find this:
49 U.S.C.A. § 13102
(16) Motor vehicle.--The term “motor vehicle” means a vehicle, machine, tractor, trailer, or semitrailer propelled or drawn by mechanical power and used on a highway in transportation, or a combination determined by the Secretary, but does not include a vehicle, locomotive, or car operated only on a rail, or a trolley bus operated by electric power from a fixed overhead wire, and providing local passenger transportation similar to street-railway service.
Hmmm . . . this presents a bit of a quandry, and I do not have time to look at this before lunch. According to the statute, "the term 'motor vehicle' means a vehicle . . . " It does not specifically say "boat" or "airplane." There may be some technical argument here over whether boats and planes are "vehicles" within the meaning of the statute, but my gut says that a common sense interpretation of the term "vehicle" will include a "boat" or an airplane. Someone may also have to go off and take a look in the maritime definitions or FAA regs, or somesuch.