Jamieson, Fausset and Brown on Genesis 1:20

Ge 1:20-23. Fifth Day. The signs of animal life appeared in the waters and in the air.

20. 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.