I don't believe water is getting in the oil, if water were able to get into the rear housing where the oil is, the oil would certainly be able to come out and run down the outside of the housing itself. That's just the way things are, if something can go in to contaminate the oil, then the oil would use the same path to run out. Are there any indications oil is running out or down the rear end housing? If there is, water may be getting into the oil.
I do know different oils do not blend together well, synthetic and regular oils don't blend (mix)together and give an appearance of contamination. Air bubbles in the oil will also give the appearance of water contamination, remember rear end lubricants are a lot thicker than motor oils are, so they will get tiny bubbles in the rear end lube and stay there unless the oil has been in the rear end and used for thousands of miles. We don't keep the oil in the rear end long enough for this to happen though.
Take a sample of the oil and put it into a clean container and let it sit for several days. This will give you a basic answer of what is going on, if it's water, it will seperate. If it's air bubbles, the air will eventually work itself out and disappear. And if it's two different types of oils, depending on how long they have been together, these will also seperate from each other after a period of time. Just an easy test, it doesn't cost anything but time.
Ride Safe,
clutchless1