But it’s not like all the water just disappears after it enters the data centres.

It doesn’t disappear as such, but it won’t be in usable form.

The reason they want to make use of the water isn’t as a heat transfer fluid to something else. The phase change from liquid water to water vapor consumes energy.

Evaporative coolers work on this principle.

So now you’ve got a bunch of water vapor blowing away in the wind, which you’re not going to be drinking.

Same thing some thermal power plants do — you probably have seen images of those nuclear power plants with cooling towers, and other types of thermal power plants will do the same, coal, oil, gas.

That being said, they don’t really need freshwater, as long as they can set up some sort of evaporation system that uses seawater for cooling, doesn’t clog up from salt or other stuff building up. The UK being an archipelago, seawater is not in terribly short supply.