Okay, maybe it's just me being dense, but...
If the airplane can bring a tankful of water to this airship and drop it on top so the airship can dribble it out the bottom, wouldn't it be just as easy for the airplane to just drop the water on the fire? If the airplane is too high for the water to hit the ground, then (subtracting the height of the airship), so is the airship, isn't it? Am I missing something?