> Favorable geography is most likely already in use.

Favorable geography ON RIVERS is limited. Off rivers, the possibilities are enormous and largely unexploited.

Off rivers it's hard to get a sustainable supply of the huge amounts of water required to make a reasonably large operation to work.

No, not really. The evaporative losses are significantly less than a water-cooled thermal power plant of similar average capacity, even in desert conditions. And compared to water extraction for irrigation the water needs even to charge the reservoirs would be modest.