The Genesis account of Creation picks up at Day Five in verse 20, and one of the first things mentioned is aquatic life. St Basil the Great poetically states that this life "ornamented the waters," and as St. Ambrose notes, this life was simultaneously created and didn't merely "evolve." Also created simultaneously with the life of the sea was birds, which St. Augustine notes generated from "air saturated with water." Both of these life forms have some merit for being created in association with the waters first, in that if you will recall back on Day One, the Holy Spirit was "hovering over" the waters "of the deep" and in a sense was incubating them in anticipation of the life to come. What is also interesting too is that many evolutionists also believe that the first life came from the waters - on that point they are correct. Evolution, like other mythologies and false religious systems, does have kernels of fact within it, and in this case they do have the sequence right although they err when they address the process - for the evolutionist, all life evolves from the water, whereas in Biblical Creation what really happened was that life started first in the water as part of God's creative plan, and as we see later land-dwelling life was created. One of the things St. Augustine notes too is that the vapor of the seas supported the flight of the birds. After looking into that statement, I found out some interesting things from science about that. Water vapor sustains the weather grid for the planet, and although at this point of time in creation weather and precipitation were not what they are now, you will recall that in the earliest part of Creation God sent a heavy mist upon the earth to moisten and sustain life. The sustaining of avian flight by water vapor therefore may have something to do with how condensation directs ocean currents, and it would have been quite an insight for St. Augustine to know that in the 5th century, but it is something that has been proven by science. So, in a sense, as condensation plays a role in the air currents, it could be said that indeed the "vapors of the sea" do sustain the flight of birds! It is amazing how scientific principles were actually defined and identified by Holy Scripture many generations before modern science discovered them, and we see more of them as the Creation narrative unfolds.
The master allegorist Origen also reminds us that there are spiritual typologies in this as well. Origen, as all the Church Fathers did, recognized the literal truth in God bringing forth life, and he takes it a step further by stating an allegory concerning the very aspects of our being, both positive and negative - God brings both forth, for the purpose of helping us to distinguish what is good from what is bad. In doing so, we strengthen the good attributes, and work on the bad ones to improve them. Both of these, as the water and land, day and night, etc., proceed from one source - in the case of our attributes, from the heart, but in the case of overall Creation, from its Creator. This is a good spiritual lesson, and also reminds us that the Holy Spirit living within us helps in that discerning process too.
St. Lawrence of Brindisi also notes something else regarding verse 20 - as St. Basil noted that this life ornamented the waters, St. Lawrence with his insights into the Hebrew translation focuses here on an original Hebrew word in this verse, sharac - the word is translated "to bring forth in abundance." The "creeping creature having life" noted in verse 20 refers to fish too, as fish are said to be creeping things because they "creep" through the waters. Noting this also, St. Lawrence quotes a Rabbi Solomon who said that "every living thing that is not from high land is (this) sharac." These things come forth in great abundance and they moved themselves, meaning that they receive the power and command of God to do so (Craig R. Toth trans., and Victor Warkulwicz ed., St Lawrence of Brindisi on the Creation and the Fall - Commentary on Genesis 1-3. Mount Jackson, VA: The Kolbe Center for the Study of Creation, 2009. pp. 73-74). It is also worth noting that although "flying things" (birds) were not mixed among things coming from the waters, their flight was nevertheless designed to be sustained by the waters - air current dictated by movements of the oceans, etc.
As we move onto verse 21, the NKJV translates this verse as being "great sea creatures" which came into being. Some historically have translated that to mean whales, and indeed whales are great sea creatures, but further study compels us not to limit the great sea creatures with immense bodies to just whales. Throughout Scripture, there is reference to a sea-dwelling creature called a Leviathon, and one reference to this is in Job 41:1, where God asks Job this question: "Can you draw out Leviathon with a hook, or snare its tongue with a line which you lower?" It doesn't take a Hebrew scholar to figure out that this is alluding to some sort of fishing, and apparently this "Leviathon" lived in the sea. The word itself has a Hebrew root that means "twisted" or "coiled," (https://en.wikipedia.org/wiki/Leviathan - accessed 12/12/2015) and this definitely does not describe a whale, as a whale cannot curl itself up or twist - it just doesn't have the body to do so. In the Byzantine Liturgy of the Presanctified Gifts which is celebrated during the Sundays of Lent, in the beginning of the Liturgy a lengthy Psalm, listed as 103 in the Liturgy but corresponding to 104 in Scripture, is sung that in essence sings praise to the Creator God who made all things. At one point in the Liturgy at the singing of this Psalm, this verse is sung - "Upon it there are great ships a-sailing, and that great beast you made to have fun." (Eparchy of Passaic, NJ, Divine Liturgy of the Presanctified Gifts, Priest/Deacon Edition. Fairfax, VA: Eastern Christian Publications, 1998. p. 9). In my NKJV translation, this verse is the same as Psalm 104:26, which in the NKJV reads like this - "There the ships sail about; there is that great Leviathan which you have made to play there." I am about to introduce you to what I believe "Leviathan" actually is, and although some have translated it as "whale" over the years, the etymology and context of its references don't match the typical description of whales. But, they do this: