>Oral tradition can be pretty good

That's very vague. Oral tradition was replaced by writing because writing is better, and the information content decays far slower. Do you have evidence of information transmitted orally across thousands of years? Best I know of is a few measly centuries, from the Iliad, where some passages obviously show knowledge of Late Bronze Age Mycenaean gear, but mixed up with later Archaic stuff.

>and it's not just the Black Sea that flooded, it's coastlines everywhere.

Which wasn't the specific claim here, which is "all flood myths go back to the Black Sea Deluge".