The way it is usually done is with heat (after all the oil is drained out). Castings of this sort are often baked to remove the moisture. You will need to take the whole pump apart and dry the crankcase completely.
Another 'trick' is to empty out all the oil and replace it with a water-excluding synthetic. There are some synthetics that will not exclude water (mix or emulsify) and some will. The milky oil is emulsified.
The 'trick' is to run the pump hot (for a while) and let the water cook off. The sad news is if the water sits in there it will rust the insides of the drive system which you cannot afford to fix.