Not sure if it actually decreases bioavailability...
What I do know is that when mixed with water, whey protein becomes a nutrient-rich soup, which promotes the growth of bacteria. Maybe that has to do with a decrease in BV (my speculation).
However, it does tend to get all smelly and nasty.
So my suggestion: take a bottle of water with volume around 300ml with your whey protein powder, and mix before you drink it.