While as I've said many times and maybe even to you also, the Flood story is a parable, and also isn't likely to even be in story form about a flood around the entire Earth as we know it, but instead as Noah knew it....
In the story, the waters cover all mountains "under the heavens" -- that is as far as the eye can see from Noah's perspective. Like a large flood. Probably even more intense than the one that happened to Pakistan a few years back, where most of the nation flooded. So, what people might call a "1,000 year" flood or a "5,000 year" flood, meaning most floods are far smaller. (though physically it's also possible the the commonplace event in Earth's history of comet impact in deep ocean could if at a shallow enough angle and an ice comet simply just vaporize a vast amount of ocean water, leading to weeks of world-wide rain, so one can imagine one of those floods at some level could happen, and it's only just normal event in Earth's history, etc....)
In the story, the flood covered the 'mountains' (really mounts, or hills basically) in the area Noah knew, so that all the local hills Noah knew (as I'd call them having been to a place with real mountains like Colorado) were covered. The tallest mountains Noah knew about, but what I'd call sizable hills.
Basically Noah would have looked out in all directions trying to see land if he's on a boat in a flood....
It'd be like....imagine if you were in the middle of Lake Michigan: you'd see only water in all directions from the top of a large boat. You could float in circles for months if the winds were right, and you'd only see water (if you survived). But that's all pretty unimportant.
The story is a parable -- that's why it's in the text. (the text has very strong parable elements included in it)
Since the story is a parable, if someone misses the parable side of the story, they have literally missed most of the meaning, so I've then tried to help people by telling them, "If you miss the parable side of the story, you've missed about 99% of the meaning."