Ideally, I figure that the whole computer (including the PSU) should be submersed in an airtight container with a small opening for a power cable. An air seal needs to be installed around the power cable to prevent contaminants (e.g. dust) from reaching the water. It's possible that the self-ionization of water (0.055 µS/cm of conductivity even for de-ionized water) will itself make it impossible to run a computer in de-ionized water, but my guess is that it has a pretty good chance to work.

It's difficult to tell how much ions will interfere with the functionality of the computer, since they can interact with each other in so many different ways.

The point of the setup is that the water can be used as a more effective heat sink than air.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram