My limited experience of hydroponics tells me that it is economical for high value crops like vegetables, selling to a European or Californian market. Thera are hydroponic greenhouses which are 100+ hectare (250 acres), but for staple crops like potatoes I think it is going to be prohibitively expensive to try to grow in greenhouses with hydroponics.
If you are going to do hydroponics for veg and work with seawater then you can just as we'll go with the Seawater Greenhouse. http://www.seawatergreenhouse.com
Possibly but that defeats the advantage of using salt water. Efficient operations need only a small fraction of the water used in traditional agriculture.
