Some say as much as 500ft.
Regardless, block off the Carribean and it'll dry up. The unsuspecting builders built it on the shoreline of the very much lowered Gulf Of Mexico coast. Incidentally, I also believe the same thing happened in the Mediterranean...may have been sectioned into 2-3 parts.
It's conceivable that the Gulf was blocked off from the ocean (much the same way the Black Sea seems to have been), and then rising waters created a breech between Florida and Cuba