פּירוש אויף בראשית 7:20

חמש עשרה אמה מלמעלה FIFTEEN CUBITS ABOVE —above the summits of all the mountains when once the waters reached the level of the mountains (Yoma 76a).

ויכוסו ההרים, “the mountains were covered (with water) hence what follows: ויגוע כל בשר “all flesh perished.” (verse 21) There was no place left to take refuge from the water.

חמש עשרה, this number describes how much above the highest mount the keel of the ark had risen.
ויכוסו ההרים, even the tallest of the mountains were covered by at least 15 cubits of water.