Well, what will actually happen is huge advances in desalinating water on demand.
The problem is....
dun, dun DUUUNNNNN
politics.
The people controlling everything want water to stay rare and valuable. Who cares how much crops die.
So, those working on desalinating water are having to do things like desalinate off shore.
One way is to use nanomachines. These machines grab on to salt ions, and then, you use a magnet to pull all the nanomachines out of the water, along with all the salt. Repeat.
Desalination is fine if it accounts for the energy costs (I know that's been getting cheaper with improved desalination tech) and the environmental question of what to do with that concentrated salt. Putting it back in the ocean can kill off a lot of sea life, if I recall, though I have not looked at the issue recently.