A couple of things:
Water cooled turbos still use pressurized oil in the bearings -- no change there.
The difference between water cooled turbos and oil cooled is that there is an additional water jacket that cools the center section. The bearings are inside the center section, but they are isolated from the water jacket. The purpose of the water jack is to prevent the heat from the turbine side from heat-soaking into the bearing center section.
The reason you have to let non water cooled turbos idle after running them hard is so that the whole turbo can cool down before the oil flow is stopped. Otherwise, the oil that is sitting in the extremely hot bearing housing after the motor is shut down will turn into sludge in the bearings and will permantly bond itself to the bearings. This is called oil "coking" (not cooking, coking as in coca-cola).
The water cooled housings do not get nearly as hot as the non water cooled housings. So, after running you do not need to wait for the turbos to cool down before shutting off the motor. Also, the residual water that is in the housing at shutdown prevents the residual heat from the turbine side from soaking into the center section and coking the oil in the bearings.
The whole idea that the turbos continue to turn 50,000 RPMs or more at motor shutdown is just not true. If that were true, the turbo would be making 15-20 psi with the motor off and the throttle blade shut.
On my T3 turbo in my old Grand National, I took the intake tube off so I could time how long it took the turbo to stop spinning after shutdown. It stopped in less than 1/2 second. In fact, at idle, the turbo would sometimes stop completely while the motor was still running. If I revved it up, the turbo would instantly spin thousands of RPM's. When I shut off the motor, it would stop spinning very quickly (1 second or so). Ball bearing turbos spin much easier, but they still stop fairly quickly after shutdown. Plus, they have ball bearings -- it doesn't hurt for them to spin. It's not like the oil instantly dries up when you shut off the motor.
Anyway, water cooled is the way to go with turbos.