While it is true that many religions share a similar flood story to that of the Jews, these cities weren't flooded all at once in some biblical fashion. Rather, the sea grew steadily over the course of millenia after the last ice age. Note that humans tend to prefer settling on the cost due to its rich resources such as fish, a d the earliest languages used the term "world" to refer to the parts inhabited by humans. Thus a flood that flooded the world could be I targeted as merely something that swallowed up homo sapient coastal cities. Even then though it happened very, very gradually.