@
pato
Back on chapter 18, when they were about to cross from the humankin continent to the beastkin continent, It was said that the seas are inhabited by monsters that are
drastically stronger than those that are found on land. The reason why the sea monsters don't attack the bridges wasn't explained though. :x
This is just conjecture from my part but, on the first page of that same chapter It was shown that the bridge was surrounded by massive whirlpools, these
probably were formed due to the bridge structure interfering with the ocean currents, and
I think that's why the monsters can't get close to It.
Now how they managed to construct those
massive bridges, if the sea monsters are so powerful that they rend the use of ships inviable, I have no idea. :u
... Or boats and ships just don't exist on that world, for whatever reason, as you said. lol