dont bother witht he water cooling, just let it warm up and cool down for a bit before shutting the engine off. unless its a roller bearing one?

fyi my turbo is not water cooled

as for the exhaust wheel, i rebuilt a turbo myself before and that lasted 6 months of abuse before i sold the car, and it was fine when i sold it.

if it really is a very tiny bit, then i personally wouldnt be to worried, so long as a blade is not cut in half etc