Desalinated water costs around $4 per thousand gallons, or an average of 40 cents per day per person.
That's pretty darned expensive compared to tap water. Too 'spensive to use for farming.
Plus you can't suck water out of the ocean to make fresh water without an intake pipe, and you can't have an intake pipe without the damned tree-huggers filing a lawsuit over some tiny fish or crab that might get sucked in.
In fact they convinced the brain-dead congress to make ocean intake pipes illegal cause they might kill plankton.

The law they passed that prevents desalination plants from taking sea water and making it into fresh clean water?
The clean water act. I sheet you not.