in which he railed a little too much against the scientific community, I went looking for more sober, qualified opinions online and found this site set up by actual archeologists:
http://www.hallofmaat.com/I posted a query on their discussion forums about the apparently ubiquitous nature of the flood myth, which can be found in African, South East Asian and Middle-Eastern/European mythology (the pan-African Nguni myth narrated by Credo Mutwa in "Indaba My Children", for instance, describes an original "golden people", a fall from grace, a punishment flood and two survivors escaping on the back of a giant fish).
The answer provided was extremely sensible . Flooding is among the most universal of natural disasters in human experience. Humans generally settle or try to live near large bodies of water. And one would expect legends to develop all over the world about particularly traumatic floods, but it doesn't necessarily imply a global flood or a common root to all the myths - that an original people carried a story about one flood all over the world.
What may have come from a common root is a proto-religion that saw natural disasters as a form of divine punishment, leading to the interpretation of many different floods in a similar manner, including, IIRC, the Atlantis myth. And given the massive disruption a major flood can cause, its also likely that talk of a better time before the flood and displacement would, in many pre-literate cultures, evolve into claims of wondrous technology and high culture, as is the case with the African and Atlantean myths.