Direct-Die cooling is a crock..
Water is very corrosive unless it's been de-ionized and all your fittings and pipes are teflon and stainless steel. Then, once it hits the CPU chip, the electric potentials and any trace contaminants in your system are going to start galvanic chemical reactions. The water will get under the CPU die and screw up the solder balls that attach the Si chip to the organic package.
The back of the die has the smallest surface area to start with, it's basically a mirror smooth surface, adding any sort of thermally conductive heat spreader is going to improve on that.
Theres a lot of good reasons as to why direct-die cooling is not done for PC's
There may be some application where is might work, using something other than water as the coolant, but that's getting pretty exotic and expensive for a tiny gain in thermal transfer efficiency.
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