On the other hand....
http://www.hydrogen-boost.com/hydrogeninjection.html
Looks as though there may have been some credible research on this, as far as using small amounts of hydrogen/oxygen added to the air intake as a fuel additive (ie, like adding a few ounces of 2 stroke oil for lubrication) to improve combustion, but you're still running on gas or diesel.
In the past, I have seen ads from reputable well-established manufacturing companies offering commercial grade hydrogen generators sized to mount on Class 8 OTR trucks for this very purpose. This was as far back as 15 years ago, long before the current flurry of activity created by the recent astronomical rise in fuel prices.
I have spoken first-hand with one gent I know at work who installed one of these Brown's gas devices on his Honda gasser, and claims he's gotten as much as a 7 mpg increase in fuel mileage. I've known the guy for a few years at work, and have no reason to believe he's an outright liar, but then again I'm not accepting what he says as the gospel truth either.
Let's just say that his first-person claims, along with info from other sources I've seen over the years, leads me to believe this might be worthy of further investigation.
But I'm certainly not going to do it via the water4gas website. You're quite right, that particular company looks to be nothing more than a scam operation. $97 for a book on how to build an electrolysis unit, when you can get the same basic info from your public library for free?
Seen the same thing over the years with electric vehicles - for every gent that would give you a realistic assessment of the limitations of EV's and attempt to dissuade people from spending their money on one if their driving needs didn't match those limitations, there were two or three waiting in the wings willing to promise you 200 miles on a charge and batteries that would never die just to con you out of your money.
Same situation here, trying to sort the wheat from the chaff.