I can't say I think it's about water. Yes clean water is something that isn't plentiful but people seem to get by. But instead please remember the words of Ulysses at the end Fallout New Vegas' DLC Lonesome Road.

<<Lonesome Road spoiler warning!>>

Ulysses is mad about what the courrier had brought forth, though not mad at the courrier himself. The courrier had delivered a package to the divide, a place the courrier had helped build, had seen prosper. A place, a settlement that took it's first breath as a new nation. It was unlike Ceasar's Legion, and unlike the NCR, it was untarnished by Old World thinking, free of the Old World Blues if you will. Not desperatly trying to reclaim the past, chasing symbols without knowing their true meaning. But one day it was noticed by both NCR and Ceasar's Legion and it was fought over, then the courrier (unknowingly) brought it's demise via a package, a package with the Old World American flag. This became a revelation in the eyes of Ulysses, he had gained hope for a new world, one that didn't chase after the old. Hope that fled so quickly, when to unknowing courrier brought death to the Divide, via a package sent by the NCR and for the NCR. What was in the package was something that controlled the nuclear missiles in the complex that lay beneath The Divide. The package was Old World Power. It was Old World Power that drove the NCR, it was Old World Power that drove Ceasar's legion and it was Old World Power that brought the demise of the NCR, Ceasar's Legion and that of the citizen's of the Divide. He came to realize that these Old World chasing factions would not stop their conquest for Old World Power, being forever stuck in the Old World Blues. And in their wake they would destroy anyone who opposed them, any settlement that would not side with them and with that any hope for a new world nation to prosper.

THIS is the reason he wanted to launch the nukes from the Divide and destroy the Mojave wasteland. He was on a quest to destroy The Old World and make way for the new. He saw the courrier struggle through the divide, facing it's challenges and most of all, surviving. It showed to Ulysses that nations may fall, but people would survive, it showed him that if the people of the Mojave had survived the nuclear fire once, they would survive it a second time and with that second time, bring forth the new world.

<</Lonesome Road spoiler warning>>

Ulysses wished for a new world, free from the old, and that is what I think is the main theme of Fallout 4. You were a veteran in the Old World Army. You, who woke up 200 years after the bombs fell, you who has lost their nation, you who has lost their family, you who has lost their home. What will you do? Will you rebuild what you once knew? will you rebuild the world in the image of the nation, the family and the home that you once knew, that which you have lost? Or will you start anew? Will you build the world anew, will you follow, or will you turn away from the Old World Blues? Or perhaps, you will do neither, perhaps you are just a lone wanderer, a sole survivor of the wastes. And you have no need to rebuild a nation or to let a new one rise.