They used water injection in fighter planes in WWII for emergency power. The problem was that once you used up the water you had also used up the engine.

Water injection, vapour injection, cracking your own hydrogen with a spare coil and using the hydrogen as a "catalyst" during combustion, all that stuff has been around as long as the 200 mpg carb with about the same credibility. If there was any truth to it the car companies would be all over it. EGR was introduced to do what the water was doing with no refilling or freezing of water containers - reducing the temperature of the combustion under light to medium loads to reduce nitrogen oxides, formed when combustion temps get over 2500 F.

And yes it did help with gas mileage, allowing a decent amount of spark advance while preventing pinging.

The phony baloney explanations about fuel injection being tuned for pollution is exactly the opposite of the truth but typical of the unscrupulous trying to baffle the unwashed.