OK - far from ideal but as a means of cleaning the turbine apparently just as effective as flying through torrential rain. Which bit(s) got cleaned is unclear but apparently power went up and temperatures went down as a result.
The water washing basically cleans the crap off the compressor section. (In the case of the RN Helicopters it is more to do with keeping corrosion in check from the salt air). This will improve the EGT margin and reduce the fuel burn. We typically see a 9-10deg improvement. The improvement in SFC equates to several million £ a year across the fleet.