Since your compressor is an older one, any one of the water passages, mating journals, or heat exchange cooling devices could be compromised by corrosion if the coolant was not always maintained properly, and leak from there into the crankcase. Especially if this only happens if the unit is running. Sometimes a threaded bolt hole into the oil sump body or block was drilled out too deeply and finally begins to leak. Sometimes you can get lucky making a repair on a badly exposed surface by cleaning it with strong solvents and then either brazing the fault or repairing with an epoxy compound. You are only trying to stop a low pressure water leak.
You could be reacting too strongly to a merely incidental amount of water contamination, although there certainly should be none, in a perfect world. Some oil sumps are protected by a breather that has humidity-absorbing dessicant filters attached. The coolant could be causing condensation inside the sump from the humidity-laden air entering the crank case area. Condensate drains on an intercooler or final cooler can not only be blocked, but maybe some genius who preceded you on the ship re-routed a drain pipe into the oil sump! Since you have already done a lot of troubleshooting to learn where your problem really is, you can't dismiss even the most outrageous possibility!