First law of thermodynamics: You can't win.
Second law of thermodynamics: You can't even break even.
Anything that breaks these two is BS.
Any time you convert energy from one form to another, you lose some.
We also need to remember that a high percentage of the energy from the gasoline is blown out the exhaust as heat, instead of pushing down the piston.
I suspect that the way these kits work is by transferring water into the intake, allowing the gasoline to burn more efficiently. Remember that water expands a lot more than air for a given temperature rise, and also that it takes a lot of energy to boil it. Therefore, it will create a higher pressure at a lower temperature, when burned along with air in an engine. The higher pressure transfers more of the available energy from the gasoline to the piston, and the lower temperature reduces the tendency to ping.
However, they're doing it the stupid way. Water injection kits have been around for years. Just introduce a little water into your intake and you could do the same thing a lot cheaper.
Has anyone noticed how your mileage seems to go up when it rains?