A datacenter on land that relies on water for cooling is using fresh water. Aside from the insane and undesirable use of fossil water or groundwater, fresh water has a much higher ratio of surface area exposed to air and volume than the ocean does.

That ratio is important because you can only evaporate off the surface, so the same volume of water would evaporate faster in a frying pan than it would in a saucepan given the same conditions.

Freshwater is also smaller than the ocean by many orders of magnitude. That’s important because a smaller body of water will heat up more than a larger one given the same conditions.

Freshwater is also generally speaking moving towards the ocean somehow in a complex process called the water cycle. It might flow down a hill into the ocean, it might drip through aquifers to the ocean and it might evaporate and fall as rain either in the ocean or somewhere else where it takes some other path towards sea level.

That last part is important because in America rivers like the ones in Colorado and California have been reduced in volume so much due to of a bunch of manmade events and earthworks that they now lose a higher portion of their volume to evaporation as opposed to flow. That’s a big deal because the Colorado for example flows south but water vapor that comes off of it is blown east. So now much more of the rivers water is going into the desert as opposed to reaching the ocean. The water cycle has been disrupted.

Freshwater is also comparatively super rare and necessary for life on land.

So if you were water cooling a datacenter it would be better to use the huge ocean with much lower surface area to volume ratio and much more volume than to use the rare freshwater that will get heated up much more by the same energy, evaporate faster, we already know can have its water cycle disrupted and all life on land relies on.

Now a person might ask “what about the sea life, doesn’t it matter?” Of course sea life matters, but the way that sea life handles an increase in temperature that’s localized to one specific area is to just go away from there, like it does when undersea magma vents pop up. When the freshwater gets too hot it all evaporates away, there’s no water and the land animals die.

A person might say: “well don’t animals die when the oceans get hot too?” And they’re right! An average increase of .5 degrees c in sea surface temperatures would lead to massive coral bleaching. But enough energy to raise the sea surface temperatures .5c is phenomenally huge. Like getting more sun huge. Because that’s what’s raising it, getting more sun. Datacenters produce so little heat energy in comparison to that level of power that it’s not a concern.