Sea level rise does not happen overnight: It probably happens over a few hundred years, at the quickest. Plenty of time to modify ports and build new ones. Everybody could walk North out of Florida and beat sea level rise. Water moves North, but the corn belt also moves North, as does the wheat belt. Parts of Canada, Scandinavia, Siberia, Argentina, and Chile can grow wheat where it is too cold now.

At the same time, some equatorial deserts get bigger. Some equatorial jungles become uninhabitable because of tsetse flies and heat. Some lakes dry up, some form or get bigger. In general, there is more water in the atmosphere, not less.

Some countries get richer, some get poorer as weather patterns change. Some are totally underwater, but not that many.

The Earth goes on just fine, but it may be very difficult for us. Probably no new megafauna evolve, because this would be too fast for evolution.