It is not as much the presence of water that will cause corrosion inside a fuel tank.
Any water left for sufficient amounts of time tend to atract fungus.
This fungus actually feeds on fuel but requires water to multiply.
The waste product of this fungus is about as acidic as car battery acid!!
We do fuel drains on our MD-11's every weekly, at most it yields half a quart of water from the entire a/c.