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I figured I would chime in on this. The boiling point of water is not always 100C. It can be less or more depending on the pressure its under. under ten atmospheres of pressure its boiling point goes up to about 180C. Also at that pressure the steam expansion ratio is less than 175 times its volume as water.
In my research into water injection on a gasoline engine it is most useful as an internal coolant. The next most useful thing it does is act as a knock suppressant allowing more boost or higher compression ratios. Lastly it can be used as a intake charge coolant to cram about 0.5 to 1.5% more air/fuel mixture into the cylinder. None of these things is extremely useful for hypermileing .
On a diesel it is a little different story when used as a charge coolant. Its documented to be useful increasing fuel economy, power output, and reducing NOx emissions. Unfortunately these effects aren't really prevalent unless the diesel engine is operating constantly over 80% of its full output. Even then its only good for an improvement of 0.5 to 2% in fuel consumption and power output.
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