growing plants using dilute sea water

It would seem that having the ability to use brackish water would open new area for food crops

Humble spud poised to launch a world food revolution
Dutch team is pioneering development of crops fed by sea water
http://www.theguardian.com/science/2014/oct/18/humble-potato-poised-to-launch-food-revolution

Tomatoes Grow Well In Diluted Seawater And Produce More Natural Antioxidants
http://www.sciencedaily.com/releases/2007/03/070319091015.htm

If we were to do a better job managing the oceans, they might produce a lot more fish. There's a sort of grain that comes from the former estuary where the Colorado river used to flow into the sea. Mangrove swamps are indirectly producing massive quantities of food, since the are home to billions of young fish. If a useful crop could be obtained from these, people might plant them rather than destroy them for aquaculture schemes.

There is a whole scheme called "seawater farming" or something like it. Enriched seawater from shrimp farms being pumped onto dryland lowlands and used to grow mangroves for fuelwood and other biomass uses. The excess nutrients in the shrimp effluent are put to good use and kept out of the open ocean. Seems like a win/win except for the cost and energy of pumping the water.
There are several common useful plants quite tolerant of saline conditions that would probably thrive with brackish water....most of these have wild progenitors along the seacoasts. Asparagus and the beet family come first to mind, and even brassicas and tomatoes can withstand some.