Buy the kit/plans/whatever, put it in your car, and report the results.
My brother got a water injection system for his old 1969 (maybe 1971?) buick skylark, with the Carter-years massively de-tuned 350 V8 because it was built in a year where the car ran best on leaded fuel but would also run ok on unleaded, but then had to be de-tuned even more due to detonation. Using a simple water injection system that ran off of carb vacuum and a not very smart computer, he could keep the leaded tune but run 85 octane unleaded without wrecking the motor from detonation. It worked great until the computer failed and filled a few cylinders with water while we were at school. We were too stupid to know how to fix that (pull all the spark plugs and crank the motor while the water splashed out the plug holes and exhaust valves) so we cranked it against the water pressure. Luckily we didn't destroy the motor but we squeezed a bunch of water past the rings and had to do an oil change right away. It was such a durable engine, it ran fine for years after that little bit of ignorance and stupidity. I think doing that with a modern motor (filling cylinders with water and cranking it) would wreck it.
But that injection system was designed to solve a real problem with real physics, ie. cooling down the cylinder charge to reduce detonation. A mild nitrous boost might actually do the same thing. But it only affected fuel economy because we could lean out the motor and advance the timing to run well on lower octane gas, when it had been designed to run on higher octane leaded fuel. With water injection we'd get 14-16 mpg, without it we'd get around 12 due to the crummy tune required. Does that count as a magic "free" 30% mpg improvement by "burning water"? Not really, no. Plus we almost lost the motor when the injection computer failed.