The more mainstream way of thinking is to electrolyze water into hydrogen and oxygen. Which requires pure water (extremely pure; trying to electrolyze water is difficult and corrodes the anodes and such as is; doing it with salt water? Currently, disastrous, and I believe tons of money is being thrown at that problem with so far no satisfactory results). That means you waste a bunch of your energy on a desal plant just to get clean water out in your desert, and this gets you an ecological disaster as the briny mess (a desal plant converts energy and salt water into plain water... and a boatload of very highly concentrated salt water of course. As the dead sea and such can attest, that stuff pretty much kills an ecosystem, whereever you care to dump it).
I did some math and it's not that much water. But, hey, _its water_. It's more than enough to provide free and plenty water to the few people that live in these places. It's not enough to start farming alfalfa or whatever, though.