[d1sxeyes]

I think in a lot of cases, there are already solutions for recovering at least the heat from the water as energy. I'm not sure about the water itself, but I think OP's point is that there's not really such a thing as 'waste water'—it'll just go back into the normal water cycle, either being pumped out into a nearby river, or evaporated up into the air.

I think the key concern is the wasted energy, as a lot of energy is used to clean the water prior to it being used in the data centre.

[20after4]

There is another problem with evaporative cooling in regards to the effect on climate: water vapor has an incredibly strong greenhouse effect. It's complicated though, because white clouds have the opposite effect: they reflect heat back into space. I have no idea how you could begin to calculate the net effect from additional humidity introduced into the environment, you would have to somehow figure out how much becomes just humidity and how much contributes to more cloud formation.

[BehindBlueEyes]

There might also an effect on fish spawning if the temperature is higher, some fish may not return to spawn until the water is cool enough or they have lower chances of survival if they hatch early due to increased temperatures. That is, assuming the remaining water flow allows for passage of fish runs.