My following statements will all be acording to assumptions that the Genesis record is true.
1. According to the Biblical record there was no direct sunlight to make a rainbow before the flood. This also corroborates with the fact that the earth was watered by mist then.
2. Living creatures lived much longer, implying the climate and other conditions were conducive to such.
3. No direct sunlight calls for a vapor canopy over the earth. Nobody knows how dense it was or high it would've extended into the atmosphere.
4. There was considerable underground water which was broken up during flood.
5. For this to happen, there had to be immense seismic activity.
6. For there to be a canopy and lots of water underground, it is likely that there were no large oceans and they weren't nearly as deep.
7. There were no high, I say high mountains before the flood, so forget 29000' deep water.
8. Something had to give below the flood waters with the areas of thin earth crust, especially where the underwater cavaties broke up to emerge with the flood waters.
8. Possibly the oceans water was less salty and present salt water life possibly microadapted to it as it slowly increased in salinization.
9. The Poles would've frozen quickly, leaving them with much more ice than is present at them. Huge glaciers would've likely shifted around doing all kinds of mass continental and oceanic excavation in the process. The Oceans would've risen slowly to their present level as the glaciers moved and melted to lower warmer locations. This would give time for creatures to migrate worldwide as they multiplied.
10. The sun showed brightly enough immediately after the flood to produce a rainbow, so most, I say most of the water formerly
upstairs is now
downstairs.
Well, there's my take/hypothesis on how things became what things are now observed. Don't ask me to prove it. Take it or leave it for what it's worth to you.