Run a car on water? Electrolysis which is slow, that would explain why the car is a water-gas hybrid since it may not produce enough hydrogen gas in time for your drive. But do you want to drive around with a tank of hydrogen in your car? Remember the Hindenberg?
I like Millenium Cell’s idea better where the hydrogen atoms are in a borax solution. So you’re driving around with a tank of soapy water, if you get into an accident then no explosion. Their main problem is the catalyst to change the soap water to hydrogen is platinum which ain’t cheap.