Genesis 1:20

Each day, hitherto, has produced very noble and excellent beings, which we can never sufficiently admire; but we do not read of the creation of any living creature till the fifth day, of which these verses give us an account. The work of creation not only proceeded gradually from one thing to another, but rose and advanced gradually from that which was less excellent to that which was more so, teaching us to press towards perfection and endeavour that our last works may be our best works. It was on the fifth day that the fish and fowl were created, and both out of the waters. Though there is one kind of flesh of fishes, and another of birds, yet they were made together, and both out of the waters; for the power of the first Cause can produce very different effects from the same second causes. Observe, 1. The making of the fish and fowl, at first, Gen 1:20, Gen 1:21. God commanded them to be produced. He said, Let the waters bring forth abundantly; not as if the waters had any productive power of their own, but, "Let them be brought into being, the fish in the waters and the fowl out of them." This command he himself executed: God created great whales, etc.

— Matthew Henry's Concise Commentary (public domain)

FIFTH DAY. The signs of animal life appeared in the waters and in the air. (Gen 1:20-23) moving creature--all oviparous animals, both among the finny and the feathery tribes--remarkable for their rapid and prodigious increase. fowl--means every flying thing: The word rendered "whales," includes also sharks, crocodiles, &c.; so that from the countless shoals of small fish to the great sea monsters, from the tiny insect to the king of birds, the waters and the air were suddenly made to swarm with creatures formed to live and sport in their respective elements.

— Jamieson-Fausset-Brown Commentary (public domain)