Their problem is, the rise and fall of sea level has less to do with the amount of water in the ocean basins than with plate tectonics and convection within the earth's mantle. If you look at coastal cities around the Mediterranean, where we have a few thousand years worth of human harbor construction, you find that at Alexandria "sea level" has risen and that Ephesus and Carthage are both inland now. All caused by crustal motion.
This suggests that our coastal cities will probably be about where they are now in 100 years - unless there is a LOT of seismic activity over the next century.