According to this Bloomberg article, about 27 percent of U.S. homeowners are currently "underwater" - that is, their mortgage debt exceeds their home value.
That's a bad choice of word for me, as I have just started reading a scary book that ought to be in the horror section, Peter Ward's The Flooded Earth: Our Future in a World Without Icecaps.
Ward, a NASA astrobiologist, describes what we might expect over the next few hundred years if current levels of carbon dioxide buildup continue, with the expected climate effects on storms and sea levels, among other things. So far I've just read the Introduction, which features Miami in 2120 as a lawless island (without even a functioning airport or bridges, due to hurricanes plus the rising sea level), abandoned by the federal government as it desperately tries to help less doomed cities hang on elsewhere. In the rest of the book, it apparently gets worse. For example, 10 million Bangladeshis could be made homeless by an 8-inch sea level rise that could hit by 2050, making this "the most likely spot for one of the greatest catastrophes in human history," basically Katrina to the nth power.
Ward is a cockeyed optimist, in the sense that he hopes some such disaster might actually prompt a change in our willingness to address these issues, though I suppose one could say, don't bet on it.