Fallout: New Vegas - Battle over Hooverdam - WATER
It's actually electric energy that is the center of conflict here. Produced by hydroplant, sure, but fresh water is not the real issue.
Mojave Wasteland has several bodies of water and sources of clean water, and to my knowledge, most of the "water issues" in that game are minimal and inconsequential to the main story.
As for FO1, water is a scarcity, hence Water Merchants, but overall, settlements usually have some basic source. Not in abundance, however.
Your Vault has very specific needs in the form of Water Chip.
FO2 is a bit different. Drought has hit several parts of the wasteland, including Arroyo. GECK is needed to save it. In reality, GECK is a poor McGuffin and the whole thing is just an excuse to have your character save your home once again, only to discover the real threat of Enclave. In my opinion, it was lacking in execution.
That aside, most of the settlements have their source of water and food. Modoc was, for example, hit by a severe drought, but in general things are looking up.
FO3 jumped the shark here.
From Project Purity's illogical geographical placement to the unconvincing world (think of water beggars) where water scarcity is a bit artificial and ham-fisted in order to make a sort of thematic unison (and fail at it). This all 200 years after the war. Of course, you could handwave and say CW was hit by severe drought too, but the game doesn't really support this.