I don't think it was even addressed; it was more along the lines of "this is the way it is, deal with it".
One thing that occurred to me when I watched it: I recall Costner's character swimming through a drowned metropolis (he can breath water...don't ask). It couldn't have been more than a few hundred feet underwater, since there was ambient light. That being the case, dry land couldn't have been too far away, since there aren't any metropolises higher than 12,000 feet; so why was it so hard to find?
I don’t need to ask how Kevin Costner breathed underwater, because had a pair of gills on his neck. How he got the pair of gills on his neck was left to the audience’s imagination. Was it a birth defect? Was it evolution?
Seems to it would have been easy to find land as well.