So these fish, since they possess the biting force to destroy steel bars, then it means that the schools of fish underwater could easily damage those ships that survived from before the calamity, whose hulls are no more durable than steel reinforcement.
Since those small fish can bite through steel bars, it becomes a very simple matter for them to damage the bottoms of those vessels.
Despite the Imperial Court overseeing the construction of many water platforms in various places, when the flood arrives, not everyone is able to board these water platforms in time.
Now, the vast majority of survivors do not live on those large ships or water platforms.
Most survivors never had the privilege of boarding a ship, so when the flood came, the majority of them relied on many makeshift boats to go out and seek a way to survive.