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.”
21 So God created great sea creatures and every living thing that moves, with which the waters abounded, according to their kind, and every winged bird according to its kind. And God saw that it was good.
22 And God blessed them, saying, “Be fruitful and multiply, and fill the waters in the seas, and let birds multiply on the earth.”
23 So the evening and the morning were the fifth day.
living (v.20) — The word “life” occurs for the first time in this verse (Hebrew nephesh). Actually, this is the word also for “soul,” and is frequently used to refer to both the soul of man and the life of animals. In the Biblical sense, plants do not have real life, or soul (or consciousness); but both animals and men do. — Morris, page 69.
face of the firmament (v.20) — This indicates that the firmament can sometimes refer to the atmosphere. Not for one moment do we suppose that there were creatures that could fly in space. It is perhaps significant that they do not fly in the firmament, but across the face of the firmament. The atmosphere can be said to be across the face of the sky. Stars, on the other hand, were placed in the firmament. — Taylor, page 65.
great sea creatures (v. 21) — The first animals specifically mentioned as the product of this act of creation were the “great sea-monsters,” [thought by many to be whales]. It is significant, however, that this same word is most frequently translated “dragon.” Evidently the term includes all large sea-creatures, even the monsters of the past that are now extinct. The frequent references to dragons in the Bible, as well as in the early records and traditions of most of the nations of antiquity, certainly cannot be shrugged off as mere fairy tales. Most probably they represent memories of dinosaurs handed down by tribal ancestors who encountered them before they became extinct. — Morris, page 69.
Animal life was not simply “brought forth” from the earth or water, as was true for plant life. The principle of consciousness was not capable of development merely by complex organization of the basic physical elements; and so it required a new creation. God had created the physical elements of the universe on the first day and here He performed His second act of true creation … The “living creature” is the same as the “living soul,” so that this act of creation can be understood as the creation of the entity of conscious life which would henceforth be an integral part of every animate being, including man. — Morris, page 69.
fruitful (v.22) — In this case, God not only declared that His work was good, but also pronounced a blessing on the animals He had created. Though not an object of God’s love as man would be, animals nevertheless are objects of His care and concern … the blessing included both a command and a provision for the continued multiplication of the animals he had created, so that they would soon occupy all parts of the world. It is interesting that a similar command was given later to the animals emerging from the ark after the Flood (Genesis 8:17). — Morris, page 70.
I think “sea monsters” or “sea creatures” in v.21 could certainly include whales and dinosaurs, although there’s no way to know for sure. The dinosaurs, anyway, may have been created on day six. Or maybe some were created on day five and some on day six.