This is an old article, but how relevant! I love that phrase- "A Flood of Biblical Proportions."
The only ("only", hah!) problem with the logic of the people thinking New Orleans might become the next Venice is this... They're already on soft land. Their levees are made of earth. ...and the water isn't just going to gently rise, it's going to be rushing in with 100+ mph winds backing it up.
In short, there's a damn good chance we'll have to revise some maps after this, and the change in voter demographics per district there won't really matter much because the land areas that will have "decreased constituency" are likely to no longer exist as land at all.