Yea but we know 200 meters is over 650 feets and it's not that massive, I doubt 1/3 of our world would be covered by that if the sea rose this much
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.