As suggested in this question, there are some who hold that the mabul did not cover the whole Earth.
I would like to broaden the scope of that question a little. What indications are there from traditional sources one way or the other on this issue?
My source: the Gemara in Zevachim (113b):
דאמר ר"ל למה נקרא שמה מצולה שכל מתי מבול נצתללו שם ורבי יוחנן אמר למה נקרא שמה שנער שכל מתי מבול ננערו שם
As Reish Lakish says: Why is Babylonia called Metzula (see Isaiah 44:27)? It is because all the dead of the flood, throughout the world, sank [nitztalelu] there. And Rabbi Yoḥanan says: Why is Babylonia called Shinar? It is because all the dead of the flood were deposited [ninaru] there.
While it could be that the dead of Eretz Yisroel sank to Babylonia, why would the dead of India and China end up there? So it would seem that the mabul only affected Babylonia and perhaps Eretz Yisroel.