You're going to have a hard time finding a high spec computer that meets your needs, because of the cooling requirement. If I were you, I'd look to industrial enclosures designed for water proof operation. (there are industrial computer enclosures) Make the system water cooled so that you can run cooling lines outside the enclosure and use an external pump and radiator; this will allow you to minimize the size of the enclosure containing the computer. You'll have to accommodate the VRMs and Southbridge, which are typically passively air cooled (but do require cooling). You might try taping off connectors and spraying the PCB with conformal coating, to reduce the damage should water get into the enclosure. Connectors can be filled with dielectric grease on the pin entry side and epoxy on the wire side to prevent water access there.
I have never tried to do this with a computer; although, the techniques above have been used by me in other applications.
What is wanted is not the will to believe, but the will to find out, which is the exact opposite. -- Bertrand Russell, "Skeptical Essays", 1928