Kevlar,
Water injection is one of the few proven techniques for using water to help your engine
but it is not used as "fuel"
-it is used to modify the thermodynamics and combustion characteristics in the cylinder
check out:
http://en.wikipedia.org/wiki/Water_injection_(engines)
it is old school - like WWII into the 50s and 60s
- many of the high performance military piston engines had it
could be used in Ecomodding
- I knew people that did it in the 80s and got some benefit (few MPG)
but engines are better now and may not be worth the cost and weight anymore
unfortunately this concept is often lost in with the HHO chaos