Well, like I suggested, you could always recover a percentage of the power used by the wave energy (or microwave device, I guess it would be called) by means of either a thermal generator and/or a kinetic generator linked to the driveline and/or the wheels (such as how some hybrids use deceleration energy to help charge their electrical system). I mean, a microwave requires, what ... at least 300 watts of constant energy? Hell, the average car stereo takes at least 200 watts just for the head unit, and even my Geo Metro's wimpy little 50-something-amp alternator can keep up with that thing at full-tilt-boogie without the slightest dimming of the headlights at idle.
See, the trick to all this is probably just a matter of energy efficiency. Not wasting anything, and trying to recover as much expended energy so that you don't have to rely so much upon any kind of a power source. Energy isn't ever consumed, of course, it's merely converted from one form into another - harnessing that converted energy after it's served its purpose in creating motion (or electricity, or whatever) and redirecting it back into the system for use again, and doing so efficiently, is the real pain.
The more I think about it, the more I'm starting to reconsider my stance on combustion engines. Y'know, maybe it'd just be easier in the long run to convert everything to electric? Makes a perfect loop, considering the technology we've got available, now. It's like this:
WATER ---> HYDROGEN + OXYGEN --- > ELECTRICITY + WATER. Repeat ad nauseum.
Start with water and some form of generated electricty - wind power, hydroelectric, thousands of gerbils on wheels, whatever. Separate hydrogen and oxygen (via radio waves, as in above article). Use reaction process of hydrogen combustion to create electricity (combustion process results in water). Use electricity to charge giant pack of lithium-ion batteries of Electristang and run down 1/4 mile strip with madman's grin upon face.
Of course, there's no 100% efficient process in place with that concept, but it's do-able, and doesn't generate any waste.
And that's all kind of getting away from the whole salt water thing, anyway ... I don't think it works the same with standard purified water...