The movie explains that the sea level has risen to 25,000 feet. There are 39 spots on earth that are higher than that, with Everest, of course, being the tallest. I guess nobody gives a shit about those other 38 islands somewhere out there in the ocean?
shareTVTropes has a theory that dry land still exists (including some continents remaining) but the characters in the film simply never ventured far enough to discover it. That would explain a few plot details but given that Kevin Costner's character is a wanderer by nature, I find it hard to believe he'd never accidentally stumble upon, we'll say, Australia.
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