Why do you give this the heading "Science versus religion"? - It's always seemed probable to me that there were actual, but localised, floods that led to such stories as that of Noah or, before him, Utnapishtim surviving by building a boat. But of course the stories get more and more exaggerated and fanciful over the years as various bards get to work on them, and work in interference by gods, and moral lessons, and stories to explain the origins of local customs, or to try to justify them, so on.