To the ancient Hebrew mind, Genesis 1:21 does not describe peaceful waters filled with whales and fish.
It is filled with sea monsters, dragons, serpents, and wriggling swarming things.
It is a place of terror.
So God created the great creatures of the sea and every living thing with which the water teems and that moves about in it, according to their kinds, and every winged bird according to its kind. And God saw that it was good.
Genesis 1:23
This isn’t just assumptions about history. The word “great sea creatures” is LITERALLY the Hebrew word for “dragons, sea monsters, serpents.”