Water at reduced pressure boils at reduced pressure – on Everest water boils at on 70oC, too cold to make tea.
By definition, even ice becomes water vapour. Almost instantly. If we have a solar pump, raising water up just 1m causes even sea water to boil – losing all salts etc..
So then we have a chain of solar pumps, to transfer the water vapour in shore – to below the hills in the middle of South America.
We vent the vapour, and it condenses into pure water. We dredge the sea bottom – and use a river outflow to wash most of the salt out.
As we play the water onto this soil, seeds swill germinate – and the desert will bloom.