There are some plants that already grow in seawater, perhaps one way to achieve that effect might be to go about it the other way round, and try to breed or engineer one of these plants into something that can be used as a staple crop? Some quick wiki searching suggests to me that a few species of such plants are edible, though most I could find are so as vegetable type plants and not used like corn or potatoes or such. I did find reference to a salt tolerant plant that can be grown for cooking oil though.