Charles Talleyrand wrote:
Hovercraft and surface-effect craft are not regulated by the FAA. How
high must one fly before one falls under the pervue of the FAA? Is
there an official definition of "aircraft" that excludes hovercraft and
surface effect vehilcles?

I'm not sure there is a precise explanation but things that can't operate out of
ground effect to operate probably could escape FAA regulation.