Fresh water is necessary for human life because it dissolves the nutrients we eat and carries them through our body in the processes of digestion and absorption. Fresh water also helps regulate body temperature.
We cannot drink sea water for two reasons. First, the bacterial count may be too high for our health. The second and main reason is that the salt content of sea water (about a quarter-pound of salt to a gallon of water) is too great for the human body.
Even though a certain amount of salt is necessary for body cells to stay healthy, an excess leads to dehydration and death.