You are correct about the implications... Simple math.
The volume of a sphere is easy to calculate: V = 4/3πr³
The earth has a radius of ~3959 miles discounting topography. Now we need to know the radius of the flood. That’s the earth radius, plus the height of Everest, plus 15 cubits (22ft). So 3959 miles + 29,028 ft +22 feet = 3959 miles + 29050 feet = 3959 miles + 5.5018939 miles = 3964.5018939 miles
If we plug those two radii in to our volume formula, we get these volumes:
259,923,241,564 miles³ for the volume of the earth.
261,008,408,332 miles³ for the volume of the earth at flood.
If we then subtract the earth volume from the flood volume, we’ll get the volume of water required to fill that space. That’s how much it would need to rain. That turns out to be 1,085,166,768 miles³of rain. Now, let’s cut that by 25% because land, mountains, etc. occupy some of that volume. All that space would not be filled with water. The 25% figure is generous since oceans, which by definition sit at sea level, cover 70% of the earth and the rest of the earth isn’t nearly as high as Everest.
That means that there had to be 813,875,076 miles³ of rain for the flood to cover every mountain. To put that in perspective, the oceans have about 321,000,000 miles³ of water. All the water on Earth only adds up to about 332,500,000 miles³.Therefore for the flood to have covered everything , the water on earth would have had to multiply by about 250%. This would be the equivalent to melting the moon Europa on the Earth.
Let’s try to put that in another perspective. The Atlantic Ocean is about 80,000,000 miles³. That means, there needed to be more water than could be contained by ten Atlantic Oceans to cover the whole Earth. If this is indeed the case as you imply, where did the 250% extra water go?
Regards, GBTG