First of all, in general it depends what kind of water is used - river, lake, ground water - but in this case it’s almost always easily accessible river and lake water, because it’s the cheapest (surprise, surprise).
This is actually much better than it would have been with underground aquifers, because those take long years to refill.
However, whenever you collect river or lake water and evaporate it, it’ll almost never return as rain in the exact same place. Partially because that’s just how it works, and partially because climate change is affecting rain patterns almost everywhere.
So if you quickly use up half of an entire river - a good amount of the water will return as rain, but away from the river, so the river will still shrink. This is bad, because ecosystems depend on rivers and lakes, including people. It also doesn’t mean new rivers or lakes will be created elsewhere. All our current lakes and rivers have been created over millenia. Depending on where all that extra rain falls - the water may not even have a positive impact, e.g., because it’ll cause a flood.
Depending on the specific geographic location, some of the water may be completely wasted, because the rain will fall into the sea. In the middle of the country (US) it’s pretty much 0%, but closer to the shore it may be nearing 100%.
There’s also the problem of some data centers ignoring ecological regulations, and dumping all kinds of waste into the same rivers they use for cooling. There was at least one case of an xAI data center seriously polluting a river.
The single biggest issue is just the scale of water extraction. Most of humanity’s water use has been slowly growing over thousands of years, with appropriate infrastructure built and ecological research conducted. Not exactly true in the US (go figure), but I’m simplifying.
Big cities obviously use a lot of water, too. But they were built and populated over up to 300 years (or much more outside North America).
Now imagine adding several big citties worth of water consumption, all in just a few years, without any ecological oversight.