Well i guess it is my turn then, to begin i will also start with an overused but valid question, where did the water all go? Where would enough water to cover the earth even come from? Maybe christians just stole from the greek flood story!
There is more than enough water currently on the earth to cover the globe if it were spread evenly; there are two ways this could have occurred:
the earth was originally a near perfect oblate spheroid and what land there was before the flood subsided into the sea; afterward the land rose again (and it is still rising), or
by applying the same power over water that was later displayed by Moses at the Red Sea, except instead of making a passage across the sea it made the water sweep over the land until it covered everything; afterward the sea was allowed to flow back to the sea.
And now that I have shown that it was possible, you need to accept my explanation as a valid possibility that should be considered alongside your scientific theorizing.
... we should be able to calculate how much water is necessary to make a layer fifteen cubits deep covering the whole surface of the earth, ...
Or conversely take the area of the ocean that is 15 cubits deep and see that there is plenty to cover the land area. Again this demonstrates that it is possible, so it should be accepted as a valid argument.
... up the sides of the mountains and so on.
Think of gravity being switched to being a force perpendicular to all hard surfaces (even vertical cliffs) and the water would flow naturally to cover everything. Switch it back to the center of the earth and the waters return to their basins of origin.