Many organisations run water cooled overclocked servers in production. I have not yet heard of any production use of sub-ambient cooling, but that would be awesome!

Liquid nitrogen cooling is a thing, where maximum per-thread performance is needed, but it provides at best 2x. There are a lot of other things to do first.

Yes and also subambient using heat pumps, but I have never seen it deployed in a data center. How would you deal with condensation?

Control the humidity, bring the temperature up if humidity control fails.

With a small heater to blow hot air over it.

That would work with a heat pump setup. Liquid nitrogen cooling is usually done by evaporating into the air directly on the processor package (as far as I know). So you would always have condensation inside the server case. Hmm, I guess a isolated duct for evaporated nitrogen to escape and heating the outside of that duct to ambient temp.

have everything that's below ambient be submerged in an engineered fluid?

Like who?