Di ba, water is being turned into fuel by breaking it down to hydrogen and oxygen. But the process takes more energy than it can produce.... for now.
Isn't it paradoxical that hydrogen is the most abundant element in the universe, yet nobody seems to make any effort to figured out a way to mine it (from space)?
Take a look at planet Jupiter. It is made roughly of the same stuff (molecular Hydrogen + Helium) as our home star. It is a gigantic fuel repository. If only we have the technology today to mine this planet, it will solve our energy requirements for perhaps millions of years.
Of course, even if we can do it, things may not be as simple as that. We must consider the so called "The butterfly effect".