One real killer for the idea of a Global flood, is a plant called Kings Lomatia, It is probably the oldest living thing on the Planet, in that it propagates itself in such a way that the original plant still lives, and is 40,000 years old !
Here is the knock out punch, it could not survive for any length of time under salt water ! I have an e-mail from the Professor who is the World expert on the planet, confirming that fact.
As to which local flood or floods gave rise to the myths, it is difficult to say, but certainly the myths are much older than when Noah's flood was supposed to happen, and no major flood of unusual proportions took place in the Middle East at that time.
So, total Myth the story is.