God calls for the waters to abound with swarms of living creatures (v. 20). God the created the 'great sea creatures' and all other living things which fill the waters, according to their kinds (v. 21). God saw that they were good, and blessed them to be fertile and fill the waters (v. 22).

The 'great sea creatures' comprised a wide range of large aquatic animals from different kinds. Some kinds are now extinct, but we still have surviving crocodilians and cetaceans. One reason for highlighting them in this verse is that many would later be worshipped by humans rebelling against God. Here, God points out that He has created all such creatures.

God gave dietary restrictions to the nation of Israel, to set them apart from their idolatrous neighbors. Here, the Israelites were told that they may only eat aquatic animals that have both fins and scales. Fish that do not have scales (like eels and catfish) would not have been allowed in their diet. Shellfish, squid, and other aquatic animals would also have been verboten. Today, these restrictions are lifted (Mark 7: 19).

The vast number of creatures, large and small, that fill the waters of the earth were formed by the wisdom of God. Even great sea creatures, Leviathan, were created by God to swim and play in the oceans.

Leviathan here is a great sea beast. The term can be used in either a biological sense, or in a figurative sense, in different parts of the Bible. (Isaiah, for examples, uses it in prophetic passages as a symbol for Satan.)

The edible fish of the Lake of Galilee, or Kinneret, include several species of tilapia (musht), barbels (from the carp family), and sardines. Matthew 17:27 notes Peter is told to cast a line and catch a fish, which would have a coin in its mouth to pay the temple tax.

God prepares a 'big fish' to intercept the prophet Jonah after he is thrown from a boat in the Mediterranean. Rampant speculation has suggested this was a whale, or an extinct reptilian 'sea monster,' but it may simply have been a large fish. One ancient fresco suggests that the dusky grouper grew large enough in Biblical times to swallow a person (Guidetti and Micheli 2011).

Pearls were prized in ancient civilizations, including Egypt and Sumeria. Pearl oysters (Pinctada and Pteria, particularly) were collected in the Persian Gulf and along India’s coastline. There is even a native species in the southern Red Sea, Pinctada margaritifera var. erythraensis. Mother-of-pearl, or nacre, (possibly mentioned in Esther 1:6) was also collected from the oyster shells.

Coral was highly prized, with Ezekiel noting that Syria traded it for Israelite goods. The Mediterranean red coral, Corallium rubrum, was used in jewelry by all nations of the Middle East and the Mediterranean, and traded to even further regions.

Marine snails (Muricidae) provided pigments for blue and purple dyes prized by the Phoenicians and later cultures. Purple fabric is mentioned as a luxury item several places in the Bible. (Soldiers even mocked Christ by placing a purple robe on Him prior to the crucifixion.)

Marine sponges had a number of uses in everyday life in Biblical times. Roman soldiers were supplied with sponges to stuff their helmets, for personal hygiene, and for drinking (Pronzato and Manconi 2008). One was filled with vinegar, placed on a reed, and offered to Jesus as He was being crucified.