The Great Flood killed all surface creatures except those on Noah;s Ark, no?
I presume certain creatures of supernatural Might who were not on Noah's Ark, managed to survive - eg ancient dragons, etc
Is there consensus how? Would a Regio boundary keep out a Divine Flood?
Or are all the current creatures of Might descended from critters spontaneously generated after the Flood?
Whatever works for you.
There is minimal discussion of the Great flood and its consequences. I don't even remember it being mentioned in any of the books - but there probably is some mention somewhere that I have missed.
So your guess is as good as ours.
Angels and Demons were clearly unaffected, so it would seem that other realms in principle should be generally unaffected. Generally because if you want to get into some really interesting history a number of Sumerian deities have fish tails...
It's really up to you. That being said, immunity to deprivation is a built in feature of the Magic Realm you need a flaw to lose, faeries tend to be able to dematerialize between stories, and demons can for other reasons. Yeah... chances are the flood did not drown them. Maybe the Magic Realm got screwed, and land-based animals of virtue got eaten, or suffer from acclimation for that year, and so on. That being said, despite all the vaunted immortality of the Magic Realm, most characters aren't created at Winter. Same for faeries. If you want such a story in your game, no reason you can't have it. Regio or not. Unless you decide God's intent with the flood was to wipe out everything on earth, creatures of might included. If he wanted that, he may very well have succeeded. And maybe in his omniscience, he chose to exclude certain regios. shrugs I suspect the only micromanagement he did was with Noah and his ark, but I'm not theologian. But ars is deliberately structured to leave God's reason up to the storyteller to make up (if he cares to).
An aquatic or amphibious creature would probably be unaffected. Might explain the enormous creatures drawn in the seas at the edges of the map - they are older and more powerful than wee young land beasties. Vast wyrms, turtles the size of islands, and the always popular tentacled monstrosities.
They aren't, Take a look at Behemoth - Wikipedia .
Leviathan and Ziz.
I avoid this by having bible chronology only be true in places where god has the power to make it true; basically just strong Dominion & Infernal auras / regiones. Everywhere else works more or less like A} the prevailing supernatural power, or B} the real world.
So big, fierce land animals do not need a special explanation. There are other ways to patch this, of course.
Related concept: The antediluvian world had cities and cultures. They might have left behind libraries, items, ruined temples and settlements, fortresses, possibly long-forgotten gods and daimons (which need not be Lovecraftian in nature).
Do I now need to read the Old (non woke) Testament?
There isn't an extensive list of preFlood cities. 
The preFlood (primeval) history is only the first ten chapters or so. I don't know that I'd even bother with more than a basic reading; the cities of the land of Nod would be far away from whatever cultures in Europe existed.
Now that I think of it, it's not clear that the continents were in the same shapes before and after the Flood.
That is certainly a modern take on continental drift, but in the medieval period nobody ever considered that the continents were ever in a different configuration, or that there even were continents. The word was invented in the 16th century, though the ancient Greeks divided the world into Europe, Asia, and Africa, though they did not have a word that translates into continents. They basically divided the world up by which direction the lands were from Greece and assumed there was water dividing Europe from Asia, then kept the names and divisions when they discovered they were wrong. The only land that was believed to have changed was Atlantis.
Well, there's an idea for a new minor theological point taken by a fervent ideolog as a heresy.
"I notice, brother, that rivers change their course in the fullness of years, and the ground is reshaped in a flood. How much more was the shape of the land sculpted by the Great Flood. Were mountains reduced or valleys filled? What rivers coursed south that now flow west? Was the Black Sea joined to the Mediterranean, or that sea to the Ocean?"
... "Heresy! Repent of this blasphemous and dangerous error or you shall be hanged for your foul teachings!"