Re: I've thought of this often
Apparently more people than just me have considered this...
Firstly, full disclosure - I water-cooled my home desktop for years and liked it. A lot. Sadly it's all aluminium parts, so I can't really extend it or update any part of it so I eventually had to retire it when things stopped being able to be bodged into place. I'd do it again, but better things to do with money now that I have kids...
Directly water-cooling rack-mounted servers means that you have to have some way of coiling the pipes up so that you can draw the servers out of the rack on rails and not have to unplumb them - that's what I mean about the headache. A teeny leak can cause havoc, and good luck getting a warranty on that kit. And when you first connect it, you'll end up with bubbles somewhere. Again, a headache.
I did mention Fluorinert, which could then be exchanged to water at the back of the chassis. Reasonably sane, but you still have the extendable plumbing issue. At least internally it can be a sealed system with no bubbles!
Yeah - water-cooled cabinets. Looked at the sheets, but never seen one in the flesh so I couldn't possibly comment more.
Low-grade heat. Ledswinger has it. There's not many uses for it, and you can't transport it far without losing the heat in it. Under-floor heating? Great. Pool heating? Great. Domestic hot water? Not so much. I don't think it _needs_ to be kept above 60 degrees all the time, but it needs to be up past there for at least a couple of hours a day for legionella (I am not a plumber, but I have spoken to many), so it adds up to much the same thing. Wall-mounted radiators need 60 degrees, or have to be massively oversized. So until everyone gets a pool or under-floor heating then we're rather stuck for what to do with it. That's why I suggested banging a heat-pump in the middle. You'll get a lower flow-rate, but higher temperature.
And the datacentres that we're located in would still go apoplectic if you started running water into the halls. That said, there's a moderately-sized DC in central London that had water fire suppression last time I was there (about 2 years ago). Maybe they'd let you hook in...