Seawater is very hard and harsh to deal with.

The intakes tend to clog with marine growth -- everything from barnacles and oysters to algae. Pipes have to be corrosion-reistant (not iron or steel). If you're using evaporative cooling, you end up with saline concentrate. If you're using dual-cycle systems ("clean" inner cooling loop, "dirty" outer loop), you have the issue of dumping thermal load into the oceans, which can affect sea life. And to boot, east-coast US seawater isn't all that cold due to the Gulf Stream -- 22-26C (72-76F) off the coast of the Carolinas, though subsurface temps may be lower. California's coastal waters are far cooler, about 15C (60F) presently, and dropping toward 10C (50F) in winter.

I would assume that they prefer to use "clean" ground water as opposed to sewage or seawater to minimize the buildup of waste or salt during the evaporation.

One would imagine the extra maintenance required would be very costly.

Microsoft has experimented/is experimenting with lowering a datacenter right into the ocean, at least in part for the ease of cooling it.

> In fact, Naval Group adapted a heat-exchange process commonly used for cooling submarines to the underwater datacenter. The system pipes seawater directly through the radiators on the back of each of the 12 server racks and back out into the ocean. Findings from phase 1 of Project Natick indicate water from the datacenter rapidly mixes and dissipates in the surrounding currents.

https://news.microsoft.com/features/under-the-sea-microsoft-...

Saltwater is very expensive to use, since the remaining salt would clog evaporators, and the salt-concentrated water is pretty bad for the environment, so can't just be discharged.