the body has a homeostasic mechanism which regulates the bodies salts.If too much salt is introduced into the body, the kidneys would try and neutralise this by retaining water. If there is not enough water present in the diet the body will reabsorb the needed amount from the surrounding cells thus leaving a deficit. The more salt IN = more dehydration. (Obviously in large doses).
This is also why, in the desert, it is advisable to take salt tablets. This keeps the bodys water inside the cells and stopps it from being wasted away on sweat