Water cooling any chip before the complete move to "flip chip" technology is kind of pointless, since the limits on the cooling are between the chip die and the package in which it is encapsulated in older chips. Also, it's not until we get to 40-50+ Watts TDP and smaller process nodes that water cooling can become cost-effective.
As for the earliest water cooling I've seen... I think that takes me back to socket 462/A systems. I'm sure someone tried it even long before then... but s462 was when many people really started to do "silly" things with their systems to get it to the highest possible clock. I remember running into random websites that showed off custom builds with completely hand-made water blocks and systems. Now those were the truly fun days IMO. Nowadays, you just buy everything and slap it together, save for hardline water cooling loops, of course - only those are a bit more of a challenge and require some DIY.