I very much doubt that there would be any gain from making hydrogen from water on board a car and burning it in the engine. In a past life I did a PhD on hydrogen fuelling of an internal combustion engine, so I had reason to research the use of hydrogen as a fuel quite extensively. Making hydrogen from water takes a lot more energy than you would recover from burning it in an internal combustion engine. Suggesting that the electrical energy from the alternator is a "freebie" is nonsense. If you pull another 20 amps from the alternator, your engine is more heavily loaded and uses more fuel.

I treat all aftermarket bolt on fuel savers with a great deal of suspicion. Whenever they try to sell them with testimonials, you know they don't have any real science to back it up. If it was a wonderful as they claim, why wouldn't Mr Ford buy the company and fit it to every new car he makes. Instead car makers spend big dollars refining their cars and engines to make them more efficient. If there was a quick cheap fix, the big car makers would use it.

Testimonial evidence is usually crap and any perceived improvements in power or economy can usually be attributed to poor quality measurements or to a good tune up when the device is fitted. There is also the normal human response from the individual who has shelled out $2000 for a device they want to believe in. When you have invested in it (financially and philosophically), you are unlikely to change your mind and admit that you have been duped.