How? Water is already a relativly low-energy compound, so I assume you are talking about using it to manufacture hydrogen molecules to use as fuel.

An external source of water is completely unnecessary for this. The idea behind hydrogen as a fuel is that you can react it with oxygen to produce water, which is a lower energy molecule. You can then harness that difference in energy to do whatever. The amount of water generated is equal to the amount of water needed to generate the hydrogen (and oxygen) molecules in the first place. It is a rechargable battery.