The answer is really simple.
Every process we have to break water results in excess heat/energy being lost to the environment rather than being stored in the Hydrogen. when the hydroge recombines with oxygen the energy given up will never make up for that original energy lost. That heat/energy can never be regained when that hydrogen is oxidized. That is entropy.
The second problem is the quality of energy used to create hydrogen. Electricity is high quality energy and very expensive. You can now make hydrogen from water with about 90% efficiency but if you have the electricity where you need energy, you don't need the hydrogen. If you have to move the hydrogen that adds more losses. If you make the electricity from coal, you loose about 60 percent of the energy, another 5% over tranmission lines, 5-15% compressing it and 50% when you run it through a fuel cell. It would be more efficient running a coal powered steam car and cut out all of the steps. It would be even more efficient to make coal gas in the car and burn it in a standard engine.
You could make the hydrogen with nuclear heat but you would loose half your energy trucking it around the country and then loose another half in the fuel cell. If you want to use nuclear power, I think it would probably be more efficient to put the power plants on the shores to run huge desalination plants to grow alge in the deserts. At least that can be turned into diesel and piped to the cities. As a bonus, you would have enough power to never have another Califoria brownout.
What is it with lose and loose that's so hard to understand?