The problem with freshwater is not that the global supply is limited, rather that supplies in some high-population areas are.

The Amazon river sends 209,000 cubic meters into the Atlantic every second[1]. That's 18,100,000,000 per day. A single human drinks about two liters (0.02 cubic meters) every day. There are 7 billion humans, so the global requirement is 140,000,000 cubic meters of water. The Amazon alone contains 130 times more than the global requirement for drinking water.

This is domestic consumption, which comprises 10% of water usage[2]. That means that 13 times more water flows through the Amazon than all of humanity uses. That's just one river.

[1]: https://en.wikipedia.org/wiki/Amazon_River [2]: http://www.lenntech.com/water-food-agriculture.htm