Sea levels can rise—they certainly have done so in the past. And land masses can subside. But sea level is sea level. If the oceans are rising they should be rising everywhere, not just some poor island nations in the Pacific.
There is a public right of way in England called “The Broomway”. Google it. It has been in use for over 600 years. Twice day it is covered by a fast moving high tide. Over the centuries many have drowned when the tide has caught them or they have wandered off of the path into deeper water. If the oceans have been rising over the last 100 years or so, the Broomway would no longer be usable.
If the Marshall Islands are subsiding, we could return the islands to Japan, who can build an airport on it. ;)