(This is my second YKTTW. Take it easy on me.) Ah, water. It's one of the most abundant substances on Earth, and it's a thing we generally take for granted. How does that saying go, that you don't realize what you have until you've lost it? Often enough, a fictional world (be it future Earth or another planet) will lack water, or at least have very little of it. Water might be "wet gold," it might be currency, it might even have spiritual significance, all due to its scarcity. Water can often serve as a metaphor for something else, like oil or semen. Examples: The movie Solarbabies takes place on a pretty dystopian version of Earth where destruction of natural resources has sent the planet into a desert-like state. The Trope Codifier, if not Ur-Example, has to be Frank Herbert's Dune series, most of which takes place on a desert planet. Water is pretty much everything. Not quite as severe as the other examples, but it is implied in Robert A. Heinlein's Stranger in a Strange Land that water is scarcer on Mars than Earth, and has spiritual value to the Martians.
Hello, Unknown Troper. You'll need to get known to lend a hand here.