Anyone who believes there was a global flood a few thousand years ago is either woefully ignorant of the evidence, or doesn't care about the evidence because of his or her religious agenda.
There's not a shred of geological evidence for a global flood. Massive floods leave unmistakeable traces, like the scars left by huge ice-age floods that scoured eastern Washington. A global flood would have left such scars all over the world. Indeed, such scars would be the most distinguishing feature of the land.
As for myths, the legend of Noah's Flood is a really neat story to tell. I suspect that the original Sumerian flood myth was told and retold, and gradually migrated to various ancient peoples. There's good evidence that there was a lot more contact among various ancient peoples than has been thought, and it's a given that when people meet, they exchange stories. This is especially so among people who don't read or write.
AlanF