To rusra02:
It doesn't track.
Traditionally, Moses was said to have receive the Torah through the "revelation", from God. The Torah would include the narrative of Genesis.
But if you saying that Shem being an eyewitness, had passed everything about the Flood on to Abraham, and then Abraham had passed to his son, from generation to generation, all the way to Moses, in oral tradition, then we would discount Moses receiving the divine revelation from God.
Abraham wrote nothing down...unless you going by the non-canonical texts, such as from the Apocrypha or the Pseudepigrapha. But if you go through oral-tradition route, what is in the Genesis still wouldn't be first-hand testimonies.
What is more believable is that who ever wrote the Genesis, had borrowed the Flood myth, from another culture - namely the Babylonians. During the 2nd millennium BCE, a number of tablet fragments about Gilgamesh and other myths (including the Flood) were found in the Near East, including the Levant, Egypt and the Hittite kingdom, in the west.
The Babylonians themselves borrowed the Flood legend (as well as that of Gilgamesh) from even older culture, in the 3rd millennium BCE - the Sumerians. The original Deluge hero was Ziusudra. The Akkadian-Babylonian called him Atrahasis or Utnapishtim