Differential fluid compromised with water?

So last year I changed out the tranny fluid and transfer case oil, but because the differential fluid was changed recently I decided that I would wait a little while longer. Well today I decided that I would change the differential fluid and when I did the old fluid was a medium gray color. I'm assuming the oil was compromised by water. Is it possible for water to enter into the differential from the vent on the cover?
It did rain like crazy this Thursday and I drove through some flooded streets. But the water was only just touching my running boards (nerf bars). If it happened it could have only been at that time. That was the only time I drove through deep water.

Any one have any ideas?