Well I don't think you understand that scale you are talking about.
For example: The city of Toronto uses a great lake for cooling for its NBA arena.
But if you wanted to use city water that you pull in and then reject back to the city to be used elsewhere. Lets assume you have a "small" data center with a 100 tons of cooling load (12,000 BTUs/hour *100) with a total of 12,000,000 BTU/hr. Water increases temperature at 1 degree F per pound of water, so (very roughly) 8 btus per gallon per degree. So lets assume we heat city water by ten degrees (that keeps us well below legionella). (insert a bit of math here I am not going to try to show in HTML): That means this "small" data center needs to pull 2,400 GPM, all the time. And the new water has to be reject its heat someplace or it is going to be pulled back into the data center having been warmed up. So at 4am when everyone is asleep and this data center is still operating, it is still going to be pulling 2,400 GPM and the water in those lines is going to get warm because no one is using the warm water.
And remember that all of this water you are piping around has to be piped as if it is potable water - because you are claiming it is still potable. That is likely to stomp on this right there. I suspect most city water managers are not going to allow anyone to inject untested water back into the drinking water supply for a city.