Off topic, but not quite true. Popular consensus is that ball bearing turbos NEED water while plain bearings don't care too much even if they are water cooled. The reason being that ball bearing turbos have a far smaller passage for the oil to pass through and is easier to burn the oil up causing coking. Plain bearing turbos have a greater oil clearance and oil can still flow through without absorbing too much heat and coking the centre section. Either way, the car should be cooled off before shutdown after any high-load driving. I prefer load load driving to get air through the rad and oil cooler to turbo timing.

Water cooling does make a turbo last longer though.

Oh, what I just typed is covered by the paper Hen posted up. Pretty interesting reading considering OEM turbo is not set up like that in S-chassis cars. I always wondered why one side of the water jacket was higher than the other.