Computer submersed in water?

Why is that? The positively charged hydrogen ions will naturally move toward the negative power supply and assuming there is exposed leads, they will gain electrons, form H2 and bubble into the air. If the actual PSU is in the air and in close proximity, it could be a source of ignition.

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.