Rising sea levels will wipe out a few cities here and there that are exceptionally low-lying and built over permeable rock like limestone that allows the rising sea to infiltrate the water table. By and large cities will build dikes, as the Dutch have been doing for centuries.
Well, first of all, sacrificing the outermost 10-20m of development to build a dike along the shore is a trivial expense compared to abandoning the city or dealing with regular flooding. But more likely the dikes will be built over the first 10-20m of what is currently the sea to avoid demolishing buildings built up against the water. That, or existing barriers will simply be raised. Whenever buildings are built directly against the sea they already require some sort of barrier to cover the difference between high and low tides and to deal with storm surge.
