The premise of 1995's Waterworld [Amazon] was flawed in that there isn't enough water in the ice caps to raise sea levels 7500m, as is established in Waterworld canon. But I wanted to see what that would look like, so here is a map of planet earth with the sea level raised 7500m, based on topographical data published by NASA.
As you can readily see, only parts of the Tibetan plateau remain above the waves. That said, while the world as we know it is gone, that's a lot dry land—an achipelago spread over an area larger than the British Isles or Japan. It seems unlikely that it would be hard to find, let alone be lost to myth or rumor.
Here's the striking intro scene from the movie, which shows the Universal Studio logo — planet Earth — being inundated. Even as a voiceover narrates that the melting of the ice caps put the world under water, it shows the world completely under water with ice caps still remaining. This could be interpreted as an easter egg, showing that the films' creators know the water couldn't have anything to do with melting ice caps.