Deltares — the Dutch semi-governmental thinktank charged with researching flood safety (and other topics) — had a “hackathon” around the topic of adapting to a 20-meter increase in sea levels. (They say by the year 2500, but that time scale may be too conservative.)
In their report on the hackathon [in Dutch], they give three different scenarios for the Dutch “Randstad” (the area where I live that includes Amsterdam, Den Haag, Rotterdam, etc.)
The top left has a “beautiful wall” to keep the sea from flooding below sea-level ground and top right has “beautiful walls” to keep the sea and rivers out of lower ground. On the bottom, you see cities above the sea (on deeper bases), which is the vision I think most likely.
H/T to LJ