Why assume a constant sea level? It has changed hundreds of meters sine 12,000 or so years ago (look up Doggerland, or any number of land bridges. Long Island Sound was a valley, Florida was 3x its current size, much of the Gulf was inhabited because the beach was a hundred miles from its current location).
If we are to survive the near term, 20,000 years or so, we must look to past climate change over the same timescale and figure out how to deal with it should it recur, and also put aside a bit for unknown changes.
Assuming that now is just like it always has been will lead to gross failure.
Remember one of Gary Larson's cartoons, “Grog- Ice closer today?”
An even better panel of Larson's is "Never put your tongue on a glacier", as it highlights an unfortunate event that is far more likely to occur than the doomsday scenarios that these kooks constantly spew.