Idiots have been saying this for 40 years.
It overlooks basic economics. The Iraq war is just a skirmish, and it costs ~10 billion dollars a month and has an economic drain of at least the same amount.
For the cost of running a serious war for 10 minutes the US can build a desalination plant capable of providing water for 2 million people. For the cost of running a war for one day the US can build delsalination plants to provide all of the water supply for Las Vegas and surrounds, the pipeline to move the water to the city and the nuclear power plants to power all of that.
Do you still really think anybody is going to fight a world war for water?
The fact is that the world has no shortage of water. The stuff covers 2/3 of the planet to a depth of 2 miles. Drinkable water falls form the sky and runs into the ocean in teralitres a second.
All the world has is a shortage of freshwater where people want it. Given that we know both how to cheaply produce fresh water and how to to move it, the idea of fighting major wars over water is a statement that could only be made by someone who doesn’t understand ecology, psychology or economics.
Minor skirmishes fought over where dams are sited? I can see that happening. Wars fought over water? Not a chance.