Using High School physics.
We use solar vacuum pumps to raise sea water up – using 1 W of vacuum. Massively cheap. We then transfer the pure water to a high reservoir, and use gravity to irrigate the fields.
So plants grow – and there is water to drink: We have to have salt licks – animals need some salt.
So the UN gives the pumps away – as it is cheaper than coping with drought and famine.