GENESIS 1:20
 

Then God said, “Let the waters abound with an abundance of living creatures, and let birds fly above the earth across the face of the firmament of the heavens.” [NKJV]
 

And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven. [KJV]

Bible Teacher:

God has told the waters to bring forth the abundance of living creatures (treasures) that should occupy the waters and utilize the ways in the waters; and the living creatures that would utilize the ways in the open heaven above the earth. And the waters have since been occupied and utilized as a distinct World or sub-world.
 

There are ways of life both in the waters and in the open firmament of heaven, and the waters here received the Word of God that authorized them to become habitation (marine world) for the ways of life under waters.

Commentaries:

The Living Creatures are living treasures both under the waters and over the waters/earth. Though the treasures of life in heaven above are far greater than the treasures of life in the waters and in the open firmament of heaven. Yet, the seas abounded with living creatures that were living treasures and vessels for the ways of life under heaven. With them, the waters of the deep and the earth were no longer void (empty).

Bible Teacher:

It's the Will of God expressed by the Word of God in the Bible verse above, for the molecules of waters under the Heaven, those molecules that formed the waters, to also form the living creatures of the waters. And that the living creatures become moving creatures that have life and movements in abundance of varieties, under waters and over.
 

At that instance, in which this express Word of God possessed the waters under the Heaven to bring forth living creatures and living things, all the waters under Heaven became "Living Waters." As a result of that, Life and Bodies of various kinds and sizes were abundant in the Living Waters then for begetting offspring of the waters and birds who like Moses were drawn out of the waters.

Commentaries:

And what's the Assignment or Purpose to fulfill by the living creatures of the waters in the Word of God above?
 

• Filling the waters, and ways in the waters;
• Flying in the Atmosphere (open heaven) and,
• Living in them.
   

These were good enough reasons or purposes (assignments) for creating the abundance of innumerable kinds or varieties of living beings or creatures in the waters and across the atmosphere. God communicated these things in His creating living Aquatic and Avian creatures/beings in various kinds and abundance.
 

Those God has called and empowered to fill the waters independently were also given their life that depends on the waters they fill. Thus, they were to have life that needs the waters as much as the waters need them too. Hence, symbiosis mutual communion or fellowship.
 

Yes, the waters need them to be filled or fulfilled, for waters should not be dry either as the dryland or as emptiness. The void or emptiness in the waters also meant dryness or thirstiness, even inside the waters. Yes, even waters do get thirsty and dry, when the waters are empty of (or useless to) living creatures. Yes, there were waters called dead sea, not because they had no waters in them but because those for whom their waters were meant to fill and be filled were dead.