3M make a non conductive solution for exactly this application. I remember reading about an overclocking rig using something like it pumped through a radiator submerged in liquid nitrogen. Was pretty cool but the solution ended up turning into a gel at low temps https://www.3m.com/3M/en_US/novec-us/applications/immersion-...

There are several. But the liquid will heat up and eventually require cooling again. Pumping it somewhere generates noise. Evaporating it creates costs and need to frequent maintenance. And a large enough tank for that not to be an issue is impractical.

Yes! Mineral oil cooled PCs in aquariums have long been a thing-to-do (but it's very messy).