Verse 21 describes the execution of the command given in verse 20.

So God created the great sea monsters: created is as in verse 1 and is used here for the first time since Gen 1.1. Sea monsters translates a noun also found in Job 7.12, where it is rendered “sea monster” by Revised Standard Version and Good News Translation. There the reference is to a legendary dragon that must be controlled so that it will not escape and cause destruction and chaos. See also “dragon” in Psa 74.13; Isa 51.9. The Hebrew word may also refer to certain living creatures such as serpents and snakes, and perhaps even crocodiles. In translation it is best to avoid a name for one particular creature such as “whale,” “shark,” or “sea lion,” and use a general term such as “big sea creatures” or “big animals that live in the water.”

And every living creature that moves: living creature is as in verse 20. Moves translates a verb that means to move about, and in the water this refers to the action of gliding or swimming. This category takes in all the rest of the living creatures of the sea that are not included among the great creatures already mentioned; in some languages it may be necessary to call them “all the other sea creatures.”

With which the waters swarm emphasizes that the command in verse 20 has been executed, that is, that the waters should swarm with living creatures.

According to their kinds means “all kinds” as in verse 11, and Good News Translation translates “all kinds of creatures that live in the water.” Bible en français courant says “and every kind of animal that threads through the water.”

And every winged bird according to its kind: winged bird translates Hebrew “bird [with] wing”; the reference is to birds that fly and therefore in English simply “birds.” See translation comments on verse 20. According to its kind is repeated in the case of the birds and again means “all kinds of birds.”

And God saw that it was good: see Gen 1.4 and verse 10.

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .