Water is the primary basis for life on Earth and without it, we would not exist here today. It is an extremely plentiful compound, yet a majority of it is unusable in its current form. 99% of the total surface and groundwaters above the ground are either salt laden or locked up as ice in the polar regions.
The oceans represent the largest reservoir of water. However, ocean water contains on average, 3.5% of dissolved salts by weight. This concentration renders this water unusable. The sun evaporates a tremendous amount of water from the ocean but since the salts are not volatile, they do not evaporate with it. This does not increase the salts concentration in the ocean though since most of it precipitates back into the ocean as rain or snow.
Most of the remaining 1% of the water moves through the water cycle as fresh water. Some of it is trapped as groundwater at lower depths and some is distributed in soil, lakes, rivers and the atmosphere. This seems like such a small amount for the population on earth but it really is not.
The annual precipitation that falls on the land area is more than sufficient to supply the entire earth's population. The problem is that this precipitation is not distributed evenly as anyone in a desert could tell you. Some areas have an overabundance of fresh waters while others have an insufficient amount.
The other problem is that with the increasing populations, it is becoming more difficult to settle in areas previously not inhabited. Increasing populations also result in a larger demand for the agriculture industry. Eventually economic growth may be limited by a resricted water supply.
It has been determined that shipping fresh water to areas in need is uneconomical and is not necessary. Desalination is becoming the solution for areas that are far from a fresh water source, but they are within a reasonable distance from ocean water or brackish ground water. Purification plants have now been in operation for many years now in areas such as the Virgin Islands, the Persian Gulf, southern Israel and even in Arizona.
The recovery of fresh water from saline water has been studied in great detail. There are several different processes which are currently utilized. The problem is not feasibility, but economics. Economics come into play because desalination requires a substantial quantity of power. In addition, the investment to build such a plant is quite high.