It seems slightly backwards to me sometimes. Water, we have tons of it on earth. It is not like plants and can grow back. It is not like coal that we have a limited supply of. It is unique in how it works. The rain falls to the earth, then it evaporates back into the clouds to refall.
It seems so stupid to say that we are running out of water. How can that be? And the icecaps are melting, so, there is drought? Yes, it can be.
Water is so fragile, it becomes undrinkable all the time. Sometimes it is from the salt that mixes in with it from the ocean. The icecaps do this. Sometimes it is from other pollutions, such as oil, that we flush down the drain.
The water also moves. Clouds pick up water, move, and dump it somewhere else. Because of this the water can be drained from places only to drench others.
It may seem backwards, but what it comes down to is not that the earth is running out of water. It is that the earth is running out of good water in the places where it is needed.