Interestingly, if I remember the statute correctly, it actually says something like "a vehicle propelled by mechanical power and which travels upon the highways." While such a definition would clearly appear to exclude boats and airplanes, I just haven't had time to do the amount of research necessary to make me comfortable with operating on the theory that "a boat isn't a motor vehicle." That seems counter-intuitive. I'm just concerned that there's another reg or code section that "ropes them back in" to the definition, for lack of a better phrase.
I'm a lawyer, but I'm not your lawyer. If you need some honest-to-goodness legal advice, go buy some.