There are already places in the U.S. that have restricted new construction because of the limited availability of water, as in southern Colorado, parts of Arizona, and in California. Oregon and Washington are looking at similiar challenges because agriculture and fisheries need water. This has nothing to do with available land but the amount of rainfall. In our southwest, many aquifers are dangerously low due to drawdown in excess of the ability of rainfall and ground seepage to replenish the aquifer. This is not junk science; it is real and happening now.
Desalination? Sure. But the first thing that will disappear in overcrowding are our freedoms. It is already happening in our cities, the most densly populated areas.
Really where does this water go, To mars?
Weather Patterns change but water does not leave the planet, it just moves. However there are those in power that will "tell" you that we are running out of drinkable water. What they mean is "give us more money (taxes) and we will "fix" this problem.
BTW did you know water is recyclable? Amazing isn't it.
No doubt, but being that we could pluck the entire population of the Earth down in Rhode Island and have space leftover makes it impossible to make an argument about overcrowding.
Unless of course you wish to deny reality.