Maybe just try putting a resistor on the fan, or even replacing the fan with a quieter one if its standard size, then just keep checking its not overheating.
I imagine watercooling a standard psu would be difficult, cause even if you could make a water block to replace the big heatsyncs in there, im sure theres probebly lots of little parts that are normally cooled by the fan, even if they dont have heatsyncs.