It's not even an externality? They just pay market price for water. You can argue the market price is priced badly (e.g., maybe prices are set by the state), but that doesn't make it an externality. The benefits/costs are still accrued by (and internal to) buyer and seller.
If datacenters are getting electricity and water at a rate lower than retail (costs passed on to residents or tax payers), and factors like noise and water pollution aren't factored in, then yes there are unpriced externalities.
It's not even an externality? They just pay market price for water. You can argue the market price is priced badly (e.g., maybe prices are set by the state), but that doesn't make it an externality. The benefits/costs are still accrued by (and internal to) buyer and seller.