Practical Explanations

One reason so many cultures have *A* Flood myth is, of course, that floods are relatively common, and many of the sorts of places people would want to live - such as river valleys - could rarely have long-lasting floods. Hearing a grandfather proclaim "The whole land was covered!" a few times when you're a kid can leave a lasting impression. Like, hundreds of generations of lasting.

However, another reason - as well as the likely reason so many creation myths mention land rising from water - could very well be due to people noting sea shells and other signs of ocean life on what is now high and dry land.

Y'see, just about anywhere on Earth which is old enough *has* been under water for a long period at least once. Leaving behind shells and fossils of aquatic animals.

Of course, this doesn't explain those creation myths in which the first people travel a long distance under ground then dig their way to the the surface of the world.