Despite what your high school coach may have told you, the culprit is not lactic acid. Lactic acid build-up has been blamed for prolonged muscle fatigue and discomfort; however, this concept is not widely accepted today.
Lets look at this: First of all, only 2.5% of the Earth's water is fresh water. Of course, making salt water drinkable is possible, but inefficient due to the Earth's population. (Its pricy!) Now, 68.7% of freshwater on Earth is in the poles, frozen.
The image shows that Earth's water resides in a very thin slice all around the Earth's surface. Distribution of Earth's Water In the first bar, notice how only 2.5% of Earth's water is freshwater - the amount needed for life to survive. The middle bar shows the breakdown of freshwater.