Rashi on Genesis
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).
Radak on Genesis
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.
ויכוסו ההרים, even the tallest of the mountains were covered by at least 15 cubits of water.