If you look at the Caribbean area on a map, it might look possible the islands to the east formed a blocking chain to the sea when sea level was lower. There would have been plenty of shallow areas and a lot more of it was open land. Some of the rivers that feed it would have carried a lot less water, especially the Mississippi due to the glaciation to the north. Maybe it dried out so they could build cities even below sea level of the time. This hypothesis would add a lot to Noah's Flood, especially since the area is much greater than the Black Sea. 5X or so.
ftp://ftp.ngdc.noaa.gov/GLOBE_DEM/pictures/GLOBALeb3colshade.jpg