It is possible to create a viable farm using seawater instead of fresh water, by growing like salicornia for human consumption and mangrove as livestock feed. Shrimp and other aquatic animals can be farmed in tanks, enriching the water with their effluent. In the past ten years or so, seawater farms like this are starting to flourish in North Africa and Mexico, turning arid coastal deserts into self-sufficient productive land. This could be applied along as much of the world's 40,000km of coastal deserts as is needed. This is enough new farmland to feed hundreds of millions of people.
In the near future, it may even be possible to genetically engineer common food plants to grow in salty water 
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