There is a very good book on the flood but I can't remember the name of it. In the book it explains that at the end of the ice age the sea levels in the Mediterranean rose and water flooded into the Black sea.. They know that the sea was historically fresh water because there is evidence of fresh water creatures and fish being wiped out as the sea water flooded in. There is also evidence of settlements now under the sea as the size of the black sea expanded. It is possible that a farmer on the shores of the Black sea saved some of his livestock on raft or a boat and that was the origin of the story that then became exaggerated.
If the whole world had been flooded as JWs suggest then all the fresh water fish would have died as the seas and lakes merged together. It is highly likely that as sea levels changed there were many floods in various places but no world wide flood.