The information that the water level actually rose by 200 meters is a lot actually. This allows us to come up with a few possible theories and is not to be put a side lightly. It proves that the world was reshaped and also that more than just a 100 years got erased. The fact the world used to look different also got erased.
Possibility 1: this was already theorized heavily. The redline is man made an the mass of the continents was used to build it. This means the water levels didn’t really rise though more it got displaced.
possibilty 2: the water level rose and land got flooded.
a few questions though. Why are all the major places so high up then? Alabasta palest was build 4000 years ago. On what seems to be the mountain of a continent. Ohara tree planted 5k years ago on a mountaintop. Dressrosa as well existed before.
And does Imu want to flood the world because water is the weakness of DF users and he wants to make that weakness abundant.
Also now the promise of Joyboy to the fish man makes sense. If 900 years ago there was more land than water Fishman were in a very bad situation. Creating a country where they are accepted on land would have been amazing for them.
Also is Luffy going to return the world to how it used to be? So water levels will go down for around 200 meters. Was the world always separated in 4 parts? So many questions.