To mostly mirror what eru said, we can easily combine hydrogen and oxygen just by burning them together.
You can also extract amounts of oxygen and hydrogen from more complex substances and turn them into water through the magic of that science we know as Chemistry. You should expect that this will generally take a bunch of energy, though.
Nanotechnology has the potential to let us construct much more complicated sorts of chemical structures. It would be really neat, but wouldn't really change how much energy it takes to separate water out of other materials, and would probably be a lot less energy efficient than standard industrial methods.
You can also extract amounts of oxygen and hydrogen from more complex substances and turn them into water through the magic of that science we know as Chemistry. You should expect that this will generally take a bunch of energy, though.
Nanotechnology has the potential to let us construct much more complicated sorts of chemical structures. It would be really neat, but wouldn't really change how much energy it takes to separate water out of other materials, and would probably be a lot less energy efficient than standard industrial methods.