This is a difficult one to swallow. Not the fly, the myth of the bag of water repelling flies. People swallow flies all the time. They don't like it, and they don't necessarily do it on purpose, but they still gulp them right down. I guess a bag of water might be helpful in a situation like that.
But what are the reasons a fly might be driven off by a bag of water. Some claim you must put bits of tin foil in the bags, or even a single penny. That doesn't sound like it holds water. Some people think the flies think the bags are large bodies of water. Others claim the flies see their own magnified reflection, and are scared away. The most popular reason is that the flies don't like the way the light shines through the bags.
Other people say this is all just a myth, and bags of water could never drive away a housefly. Faced with a big tasty meal in the same vicinity of the water bag, I wonder if the fly would really want to leave so easily. There are many who doubt this whole thing. What do you think?
You can find a more complete article about this here: Can A Bag Of Water Keep Flies Away?