Just in case any of you aren't already aware of it, there's extensive evidence that the Mediterranean partially or completely "dried up" during the Miocene (~6 million years ago) and then rapidly flooded again.
1. This is neat information: it's worth sharing. (I'm glad I ran into a discussion of it not long ago, because it helped me understand the sort of thing in this comic. Come to think of it... was it mentioned in a what-if?)
This is referred to as the Messinian salinity crisis: http://en.wikipedia.org/wiki/Messinian_salinity_crisis
I'd assume that's a significant part of the inspiration for this storyline.