Kael used a mod of the SDK where the unit would be declared a land unit that can move over water and impassable terrain whenever the unit has the ability to move over impassable land (as shown in
this tutorial). This works well, and is pretty simple, but has a major drawback. It is not intuitive to differentiate between these "floating" land units and normal land units. For example, if you wanted combat bonuses over the floating "move anywhere" land units, you can't just make a domain combat mod, since they're for all intensive purposes still land units. Also, if you wanted the floating unit to act as an air unit like a transport ship, the domain of the units that ship can transport would have to be DOMAIN_LAND, which means that a "floating" unit could transport another "floating" unit (effectively loading a bomber "floating" unit into a transport plane, which probably isn't the desired effect, at least it wasn't in my case)
The better option, IMO, is to create a new domain. I've done this with the early versions of the Starcraft mod. The units move and behave as I expect them to, but I haven't yet checked out how the AI handles the new units, nor figured out this small bug with the "floating" unit gets a health bar as big as Montana on the game screen. My plan, however, is to test and figure out all these things and put a tutorial in the tutorial section, if no one else has done so by then.