Really though, if you add in the idea of a god being involved, then really anything goes.
I just wanted to emphasize this.
If you accept the laws of physics (as we understand them) as absolute, then you can rule out the possibility of God simply by saying that something cannot be created from nothing. Therefore, in order to even consider the possibility of God, you must first accept that the laws of physics might not
be absolute. And if you can accept that the laws of physics might (even hypothetically) not be absolute, then you cannot use the laws of physics to disprove God or any religious story. Because if there is a God, and He is all-powerful, then we really have no idea what He can or cannot do - in fact, I think the idea that there are things which an all-powerful being cannot do seems illogical in and of itself.
As it applies to the flood myth, people can say that a worldwide flood is impossible. But, bottom line: an all-powerful being could create an infinite amount of water to flood the world with and then spontaneously erase that infinite amount of water from existence on a whim. He could then erase all evidence of that flood, just because He felt like it.
The only way you can disprove the flood myth using the laws of physics is if your basic assumption is that there is no God, or that God cannot break the laws of physics. And if you're making either of those as a basic assumption, then, well, why are you participating in the debate?
Sorry for the tangent. I don't mean to be obnoxious, it's just that this thread has touched on a pet peeve of mine.