Depends on where it's coming from and who else needs water. If the water supply being drawn from was collected from rainwater initially and there's still enough for local residents to also have water, then sure, it's probably sustainable. But if the water is coming from an underground aquifer and there's otherwise not enough rain in the region to go around then it's a finite resource.

> It's gonna come back down as rain and refill wherever datacenter took it from

It's going to come back down as rain, yes, but there is no guarantee it will come down in the place you need it. Fairly minor variations in surface temperatures and evaporation can have a large effect on local rainfall.

Yes, I live in an arid western state. It's obvious that you don't from your naive question here.

In many states it doesn't matter, but out in the arid west, water is constantly a crisis item.