It was an idea that got tossed around when "shooting the shit" in overclocking circles for several years in the 1990s, there wasn't really a necessity for it until we were forcing CPU into the 30W plus regions. The initial more serious experimentation with it came in late 98 or 99ish with attempts to get mendocino celerons to break through the 650-700mhz region, and were replicated by some in the Athlon camp to get to 1Ghz before Intel or AMD officially did.
There is a bit of a memory hole for the two years where amateur watercooling was making it's biggest strides, due to incomplete web archival from archive.org until later noughts, and forums being on server side perl scripts and custom software rather than the now ubiquitous vbulletin and simple machines forum software etc. Also for "web 2.0" a lot of sites basically had all their old stuff wiped offline and started fresh.
There are still some articles on overclockers.com from the end of the amateur era, then there was a bunch of custom parts available commercially from small startups, and it was still pretty much roll your own with combinations of those, then the larger cooling names began introducing products, and then we were into the complete systems and then on to sealed AIWs.
Anyway, if you want a "toy" watercooling setup for 1.5Ghz and lower, put one on you K6-III, there's a surplus place got a bunch of "Silent Stream" kits cheap that weren't well reviewed for high performance use (2Ghz plus) in the day... https://www.a1parts.com/surplus/index.html
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.