How to Run Your Car on Water
If your car burns gasoline, it won't burn water per se. However, water (H2O) can be electrolyzed to form HHO or Brown's gas. The HHO is added to the engine's intake, where it mixes with the fuel (gas or diesel), ideally leading it to burn more efficiently, which should cause it to produce less emissions. Your vehicle is still using its normal fuel so you will still be buying gas or diesel. The reaction simply allows the fuel to be enriched with hydrogen. The hydrogen isn't in a situation where it could be explosive, so safety isn't a problem. Your engine shouldn't be harmed by the addition of HHO, but...
It's Not So Simple
I don't want to discourage anyone from trying the conversion, but I would advise taking the advertising with at least a couple of grains of salt. When I read the ads for converter kits or instructions for doing the conversion yourself, I don't see a lot of talk about the tradeoffs involved in doing the conversion. How much are you going to spend making the conversion? It looks like you can make a converter for about $100 if you are mechanically inclined, or you could spend a couple thousand dollars it you purchase a converter and have it installed for you. How much is the fuel efficiency actually increased? A lot of different numbers are tossed around; it probably depends on your specific vehicle. A gallon of gas might go further when you supplement it with Brown's gas, but water doesn't spontaneously split itself into its component elements. The electrolysis reaction requires energy from your car's electrical system, so you are using the battery or making your engine work a bit harder to perform the conversion. The hydrogen that is produced by the reaction is used to enhance your fuel efficiency, but oxygen also is produced. I wonder whether the oxygen sensor in a modern car would interpret the readings such that it would cause more fuel to be delivered to the fuel-air mixture, thereby decreasing efficiency and increasing emissions. While HHO can burn more cleanly than gasoline, that does not necessarily mean a car using enriched fuel would produce less emissions. If the water converter is highly effective, it seems to me that enterprising mechanics would be offering to convert cars for people, who would be lining up to increase their fuel efficiency. I haven't seen that.
The Bottom Line
Can you make a fuel from water that you can use in your car? Yes. Will the conversion increase your fuel efficiency and save you money? Maybe. If you know what you are doing, probably yes.
If you have done the conversion and have personal experience to share, I'd love to hear from you, or you can post a comment so others can read what you have to say.