The dinosaurs drank it. And so do you!
There is just so much water on the planet and it's been here for a LONG time. Our lives depend on water - clean water. We can't live sustainably in places where there isn't enough drinkable water.
Safestars.org has a good overview of the water cycle, along with experiments for kids and lesson plans for teachers. URI Outreach Center also has some curriculum materials about watersheds and water quality.
FYI, This website categorizes shorelines, beaches, coastlines, riverbanks, marshes, wetlands, etc., only under Water* (so to speak), rather than Land, since Water significantly impacts these lands.
*In fact, some day these water-bordering lands may be literally under water if climate predictions are accurate....
Conservation Law Foundation
This is E 7 from S 3 of Bill Nye the Science Guy tv series. 7ajjh., Aired on March 1995.