Any increase in temperature, which benefits evaporation, decreases the efficiency of running the data center.

This is the vapor pressure curve for water. Under constant pressure and saturation of the air, the evaporation rate as a function of temperature is proportional to the vapor pressure curve. As you can see it is an exponential relationship.

Now you have to offset that with the energy it takes to pump the water around, the increased costs for equipment that can deal with salt water, the increased costs to deal with flooding risks…

You know what you could do instead?

https://en.wikipedia.org/wiki/Salt_evaporation_pond

All it takes is a flat basin and a dam with a gate to let in water and then block it away. The sun will do the rest.