It's possible to use waste heat to evaporate water, to make drinking water from salt water, but it's unpractical to build datacenter on a shore, or pump salt water uphil to a datacenter.

Seems like Microsoft and Google are trying to make it work.

How Oceans are Being Used to Cool Massive Data Centres [2017]

https://www.vice.com/en/article/how-oceans-are-being-used-to...

Alternative Water Cooling Sources for Data Centers [2019]

https://www.ashrae.org/technical-resources/ashrae-journal/fe...

> While the temperature of surface ocean water can fluctuate seasonally, it can still be a valuable source of cooling. In hot regions such as the Middle East, surface seawater as high as 90°F (32°C) is used for condenser cooling with conventional systems. This is because even the higher temperature water is more efficient than air cooling and using potable water for evaporative cooling is not allowed due to water use restrictions.

Feasibility study and multi-objective optimization of seawater cooling systems for data centers: A case study of Caspian Sea [2021]

https://www.sciencedirect.com/science/article/abs/pii/S22131...

https://doi.org/10.1016/j.seta.2021.101528

Evaporating water is not the best way to desalinate water as it's inevitably energy intensive (though waste heat makes that less of an issue). It seems unlikely that datacenters will produce enough waste heat to supply populations with drinking water.