It's also worth noting that even with a very efficient process it takes a lot of energy to split water into hydrogen and oxygen. In addition, the storage/delivery of the hydrogen takes a lot of equipment and energy, the hydrogen itself tends to escape easily and corrode equipment, and converting the hydrogen back into energy does not happen with 100% efficiency.

Hydrogen has a few key uses but it is really not a good replacement for most uses of fossil fuels because of these issues.