The problem is, water is not flammable as we all know. But it's two components Hydrogen, and Oxygen are very flammable. The hindenburgh was a hydrogen balloon, and look what happend to it from a spark of static electricity.
To get Hydrogen out of water, you have to use electrolosis, and that takes a ton of energy to break the Hydrogen/Oxygen bond. It takes more energy to break that bond that is created from burning the hyrdogen.
Now if you have either a cheap source of hydrogen (fossil fuels are a good source) or a cheap source of electricity, you can make the hydrogen car work. In Iceland it is working rather well because they basically have limitless free electricty that comes from geothermal generation. So in that instance, they can make hyrdrogen cheaply and basically give it away for use in automobiles.
But just pouring water in your tank and breaking it up there will just not work.