In this video, Sylvia Earle talks about the importance of protecting our oceans and the extend to which we've already depleted the oceans.
An in-depth and interesting look at the history of the supply of water. California in particular is discussed in relations to water supply and demand.
The total amount of water above, on and in Earth is not very much. Since most of this water is not usable, the usable parts must be taken care of.