“Can you use Salt water or does it need to be fresh water?”
That is a very good question, which I don’t know the answer to.
Using saline, brackish water is being done some. It has additional challenges in preventing scaling and requires different chemistry. Down deep in many places, this is available and not a source of drinking water. This is not as salty as “sea salt water” but would be considered a diluted version of normal sea water, without the organics.
http://stateimpact.npr.org/texas/2013/03/28/drilling-boom-spurs-a-rush-to-harness-brackish-water/
Combined with the recycling of hydro frac water programs, it would make a difference to areas with extreme water limitations.