Good point which is something those that have a hard time with the water being 15 cubits over the mountain tops need to remember or investigate. The mountains at the time of the flood weren't as high as they are now.
The Bible does not say that the water was 15 cubits over the mountain tops. Its a wrong translation.
Water was 15 cubits deep in total, it also covered the hills.
Translation is always an intepretation, we must realize this.