There is fresh water under that desert—quite hot though.
Yeah, there are a couple of massive underground aquifers where the ancient lakes in the sahara used to be. the Libyans have built a canal to haul a river of underground water to their coastal cities—from one of these aquifers.
That water is fossil water. There might be at most 100 years supply—but likely only half that much. It doesn’t renew with rain like the oglala aquifer in the US midwest—because there is no rain in the sahara to renew the aquifer.
The israelis best desal plants are doing the job for $500@acre foot. They send the desalinated seawater to their cities and then clean it up once more before sending to their farms.
By contrast the newest US desalination plant is the poisiden plant in san diego. It desalinates for $2000@acre foot.