Done my research, now I need a review. 20 GPU cracking cluster.

[Flomac]

But the concept of watercooling is having a low temperature on the spot (GPU, CPU) and does not guarantee a cool system over all, because at the other end the heat still has to dissipate into a radiator. It's tricky to build, not very reliable and expensive.
Air cooling instead is no rocket science at all. Just use reference design cards, throw a bunch of cooler in the case and you're done.

Watercooling does not mean no system fans. I thought  that is a safe assumption.

I never said you get rid of system fans with water cooling. I just meant everything gets more complicated, expensive and, the worst part, more unreliable.

As someone who deals with IT risk management on a regular basis I take every component and weigh its risk of failing to be disastrous for that certain system.
In a usual server the loss of one, two or even three cooler is no big deal. In a multi-GPU setup the failing of a GPU cooler is not nice, but then that card will clock down and maybe gets destroyed. Very unlikly to take down the rest of the rig.

With water cooling everything chages. Every seal broken is a potential hazard. The pump could fail, so you need more than one. But every pump more ramps up the chance one of them fails. So you need a contruction where each GPU is being served by more than one pump. That means more seals, but then more seals ramp up again the chance of one failing. And, as it has been explained by epixoip, one broken seal means death of the system.

With such a huge water cooling setup we're talking about several liters of water circulating. If water is leaking it might exit that system and reach other systems. Spilled water can conduct to a shortcut and there is also a chance that a system catches fire, affecting the other systems too.

At all, the risk of failing in a certain time is getting near to definite. That alone is a reason not to use water cooling in HPC enviroments. Or datacenter. Or simply anywhere someone has to give out a certain warranty for such a system.

If water cooling's been used in HPC it usually looks like this:


Custom made, almost no seals, way lower risk.