We learned in parashas Noach that all the inhabitants of the earth would be wiped out in the Flood, with the exception of Noach and his family and a selection of animals. Noach would become the father of mankind. However, in this week’s parsha, Lech Lecha, we encounter Og, king of Bashan, HaPalim - "the fugitive", who survived the Flood. Where did he come from since, except for the Noachs, all mankind was supposed to have drowned in the scalding hot waters?
Rashi (Bereshit 14, 13) explains that the "Palit" (fugitive) has survived the war of the four and five kings as most of his people were killed. He also brings the Midrash you have mentioned, that he survived the flood by hanging on to the arc, but that again is a Midrash and should be understood as such.