I'd assume that most datacenters use evaporative cooling, since it does consume water directly and is pretty cheap. Some web searches on the topic were swamped with companies trying to pitch their solutions (as if I'm in that market??) but this[1] link looks... slightly less suspicious.

Dealing with waste hot-water sounds like a PITA. Piping it straight back to the reservoir would need a dedicated pipeline, probably with insulation so you don't bake everything around it. Harnessing the excess heat on-site would be awesome, although I'm sure datacenters would already doing this as best they can to recoup electricity costs.

[1] https://www.datacenterknowledge.com/cooling/evaporative-cool...