200m has risen.
Meaning, all land of the old continents ranging from 0m to 200m is under water.
If we are going by real life, about 1/3rd at least of the sunken lands, will be in the 100m to 200m range.
Anyone diving just a little bit would discover the old cities, old constructions etc in this range, even if we are to believe 200m is unrealistic.
Meaning, just random fishermen, marines, pirates etc would've discovered this already.
Also, fishmen enslavement only factors in if they went to the surface. Nothing prevents them from swimming around 100-200m deep.
Plus, strong fishmen like Jinbe wouldn't care either way.
This writing makes no sense.