41 The Hebrew text again uses a cognate construction (“swarm with swarms”) to emphasize the abundant fertility. The idea of the verb is one of swift movement back and forth, literally swarming. This verb is used in Exod 1:7 to describe the rapid growth of the Israelite population in bondage.
42 The Hebrew text uses the Polel form of the verb instead of the simple Qal; it stresses a swarming flight again to underscore the abundant fruitfulness.

In other versions

ESVEnglish Standard Version

And God said, “Let the waters swarm with swarms of living creatures, and let birds fly above the earth across the expanse of the heavens.”

NIVNew International Version (2011)

And God said, “Let the water teem with living creatures, and let birds fly above the earth across the vault of the sky.”

CSBChristian Standard Bible

Then God said, “Let the water swarm with living creatures, and let birds fly above the earth across the expanse of the sky.”

NLTNew Living Translation

Then God said, “Let the waters swarm with fish and other life. Let the skies be filled with birds of every kind.”

LEBThe Lexham English Bible

And God said, “Let the waters swarm with swarms of living creatures, and let birds fly over the earth across the face of the vaulted dome of heaven.”

KJV 1900King James Version

And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven.

Public Domain