Water covering the tops of mountains is just a little hyperbole that tries to convey the magnitude of the partial extinction event. In scripture "the mountains of Israel" also refers to the big shots of the community. Therefore, water covering their highest mountain could also indicate that all the well heeled big shots drowned. 10 feet of water would have done the trick, especially among desert dwellers who probably couldn't swim for a minute much less a month.. They really didn't need to exaggerate much.
That remains your own interpretation. No one knows what Noah saw to tell that it's actually a hyperbole.
Everyone knows that there isn't enough water on the planet to cover the top of any mountain in a flood..
It can only be figurative or hyperbole, or both, to make clear the lessons of the past.