I think for a lot of people, their deeded land is in eventually in terms of lat/long, and if the water swallows their land or their land falls in the sea, they're pretty much SOL. Depending on the rules of their locality, they may keep ownership of the land that's now underwater: it may effectively cease if underwater land is not subject to private ownership, or it may continue but not be of value because you may not be able to exclude other people from the land (or the waters above it) or develop it.
For some though, the deed may be defined in terms of the coastline, and then they're going to have an interesting legal battle. But this isn't without precedent; coastlines and waterways change and things defined against them adapt.
That portion of the Northeast quarter of Section X, Township Y North, Range Z East, W.M., in Blank County, Washington, described as follows:
The Southwest quarter of the Southeast quarter of the Northeast quarter of said Section. Except for some bits that aren't relevant.
That's not in terms of lat/long per se, but the section and townships are effectively equivalent to lat/long. If the shoreline moves, my property doesn't. Technically everything is relative to this stone ... https://en.wikipedia.org/wiki/Willamette_Stone
For some though, the deed may be defined in terms of the coastline, and then they're going to have an interesting legal battle. But this isn't without precedent; coastlines and waterways change and things defined against them adapt.