Others have tried this as well, and if you could get a large enough supply of H2 out of it, then yes, you could run a larger engine. I have played with a lot of different devices and there are some that work fairly well. None however that I have played with would actually run a car without needing
several of the devices running full tilt. Most of them would (and do) work good for either offline H2 or hydoxyl production that you can store, or use in a lower volume application (like heating or a stove to cook with). They are VERY good at heating the ammonia for an absorption style freezer or fridge (10 watts versus 300-600 watts of resistive heater elements).
Another good place to look:
http://waterpoweredcar.com/
and their sister site:
http://hydrogengarage.com/home.html
You can get parts or even a kit through the second link, and it will generate either hydrogen or hydroxyl, but don't bet on running your normal sized car from one of these devices directly. Remember, there is no such thing as a free lunch, these devices may only take a few dozen watts, but they won't produce enough hydrogen to run even a 50 horsepower engine directly (without storing what is needed for the run, first). Any engine will quickly use up what one of these devices can create in real time. A 50 horsepower engine, at 100% efficiency, would need a minimum of 37,400 watts of energy to get running at rated power. And that is just for a 50 HP engine (like an older VW bug engine). Once you get into the range where the big V8s are, it takes much more energy in. You are not going to run an engine that needs 100KW input, with 100W. Notice how most of these sites call them "boosters" since they know that one will not run a car by itself, but
possibly help the gas mileage during regular gasoline powered use.
They show you how to build your own, try it
http://waterpoweredcar.com/hydrobooster2.html . I have made a few, as have a few friends of mine. They can be used for small jobs, just don't expect miracles...
Loup