Mineral oil has been used for this before, I think some other oils also work. From what I've read, they work fine, but swapping out parts is a hassle because the oil is tough to get off of contacts.

Distilled water theoritically works. But it will quickly ionize, which will kill your system.

A friend of mine bought the first 1GHz system back in the day. I think the company's name was Cryotech. It had an overclocked Pentium that was encased in a chiller. The case weighed a ton, and sucked up a lot of power, because it contained a small fridge compressor. It took the CPU enclosure down to -40F before it would power on. More of a novelty item than anything else, since it didn't take long for the aircooled systems to catch up.