Actually, no, the KJV translation of Genesis 7:20 is just misleading. The verb "prevail" there in the original it also means "surpass". So the sentence actually said it rose, surpassing the highest mountains by 15 cubits (about 7m or 22ft).
I mean, WTH, the waters rising 7m would be the most stupidly ineffective genocide ever. I mean, forget mountains, 7m doesn't even cover a moderate mound. And then you wouldn't even need a boat, you could just climb on a tall tree or on the roof.
Hell, there were palaces and temples back then which had a higher foundation than that.
I mean, then you'd probably see the next verses being, "And lo, everyone huddled on top of their temples and towers, and were mightily pissed off at the water damage to their property, but otherwise only a couple of guys locked in a dungeon did perish. And everyone agreed that the god of the Jews must be such a prick just because he's so impotent."
ETA: I mean, wth, Noah's instructed to build a boat 30 cubits high, or about 14m. Allowing for even minimal elevation above sea level to start with, say, 1m or 2m, if the waters roes 7m, that thing wasn't even floating at that point, but still had the keel on the ground. Why not build a platform with a fraction of that wood, then?