Bro don't underestimate the outcome:
conservative estimates suggest that a sea level rise of 200 meters could submerge more than 20% of the current land mass, possibly as much as 30% or more, depending on the topography of the affected areas.
The bigger Problem: It is estimated that around 40% of the world's population, or around 3.2 billion people, live near the coast. This figure varies depending on the definition of "near the coast". Some studies refer to a radius of 100 kilometers from the coast, while others use a radius of 50 kilometers or even just 10 kilometers.
Oda choose a good rise of 200 meters. So we can assume that around 40 percent of the Ancient OP World population died, it could be mor the 40 percent due to economic and agricultural problems after the flood and we shouldn't forget that a 100 year old war was fought.
In my opinion I assume that 60-70 percent of the Ancient Population died.
Good point.
I think that the mass flooding killed 30-40%, and then another good number died during the war that followed. This assumes that the Void Century STARTED with the mass flood.
IMO, the World Government caused the Ancient Flood with the Ancient Machines, and then blamed it on the Ancient Kingdom.
After, the entire world went to war with the Ancient Kingdom, only to realize that they were the good guys when the war ended, and that the World Government were the ones who had caused the flood and war.
So, the flood started at the beginning of the Void Century, led to the war, and the Void Century (a war that destroyed history) finally ended with the war's end.
This would explain a lot, because if that led to 100 years of war, I can understand why the Gorosei and WG really want to avoid that from happening again.