A lot of hydrogen is tied up in water already, or in other molecules. So you would have to disassemble other stuff to get the hydrogen. Hydrogen is extremely abundant in the universe but not on Earth. Most of it is in stars. You also need energy to make water. You have to heat oxygen and hydrogen to get them to bond. It's not helpful if it takes too much energy for a little payoff. Sure you can create a little vapor with a small flame but to create a large reservoir you would need more energy. Pretty much all energy except for a few sources work by boiling water. So if you have enough water to run power plants then it's probably easier to recycle that water than to make new water.