One of the theories of the origins of these flood myths (which I find reasonable) is that they are the rememberences of the factual Great Flood of old. Once, the Meditteranean Sea wasnt. What is now Gibraltar was large wall of rock. It formed an enormous damn keeping out the Atlantic. That wall failed, and when it did the ocean came rushing in destroying the habitations of men who lived there. A great wall of water ruined everything in it's path. These tales may be the results of the telling and retelling of that event.