An area might run out of water, but the planet won't. The planet would run out of literally everything else before we run out of water.
It takes water to make a human. You will pass the point where you can't expand the population before you get to the point where you can't maintain the population that already exists. If something does decrease the water supply, you will in a short period of time have a smaller population to support. Such is the nature of carrying capacity.
Sure, but practically speaking there is a lack of potable freshwater in certain places. You can’t just stick a straw into the Indian Ocean and glug away.
It's a simple analysis - which is easier: bringing more water to where it is being consumed or moving the consumers to the water. In a wealthy area with a lot of stuff going for it, maybe a desalination plant makes perfect sense. Everyone in the city drinking non-potable water does so because they have judged it more practical than moving to a place with better water infrastructure.
If you get a million people to build a city in the Sahara and don't build any infrastructure to get water to this city, of course they are going to have water shortages, but this does not suggest some global water crisis nor is a limited birth rate going to fix the problem. Likewise if someone sticks their head in a plastic bag they may run out of air, but that doesn't mean air is any less abundant. There are some resources of which there is an actual scarcity such as arable land and energy sources, but water is not one of them.
There's certainly a regional crisis that will only probably get worse as climate change reduces snowpack in the Himalayas, the source of most water in India, China, and SEA. The problem with moving is threefold; moving to countries without water scarcity legally is not a realistic option for most Indians, cities are highly sticky, and new cities are incredibly hard to set up and set up well.
In fact, China is already considered to be suffering from water availability issues, and while this still happened with a one-child policy it almost certainly would be worse had Chinese population growth had the same trajectory as India's. (This is not an argument for the general good of one-child policy, and I do not endorse such a thing.)
The himalayas are the source of water for the region because it is abundant and cheap. Infrastructure to bring water in from more distant sources, desalination plants to generate more fresh water, wastewater treatment plants to recover more water, and changes to water use such as different agricultural methods which conserve water are all options to increase supply.
On the demand side, if moving is not legally or socially acceptable, what is the difficulty of changing the laws or culture? If cities are sticky, move the things that attract people to those cities elsewhere. If new cities are difficult to set up, how difficult is that compared to modifying an existing city?
there's a big difference between "there isn't any water" and "we won't take actions to get water." Now you may be saying "but those things are hard and expensive" to which I will respond "yeah, providing for the needs of 20% of the world's population is going to be hard and expensive," but on the brightside 20% of the world's population is an incredible resource if utilized properly.
Of course there's "technically" enough water, but that's academic pedantry at that point. There's technically no such thing as peak oil either, but there is a point where it becomes economically infeasible to produce more oil, which is what the point of the reserves statistic is. Reserves aren't all oil known in existence, they're all oil that is known to be feasibly economic to get.
The problem with new cities is generally trying to move employment. Unless there is a specific reason to move employers tend to like clusters of other employers. Most planned cities without a specific employment reason either fail or become big suburbs.
Making it easier to move to other countries is not exactly within the realm of possibility, given that India is not in control of how the US makes legislation and pressure would pretty much result in backlash that would probably make the situation worse, not better.
It takes water to make a human. You will pass the point where you can't expand the population before you get to the point where you can't maintain the population that already exists. If something does decrease the water supply, you will in a short period of time have a smaller population to support. Such is the nature of carrying capacity.