Targum Onkelos: And the Lord said, Let the waters generate* the moving creature (having) life; and the fowl which flieth over the earth on the face of the expanse of heaven. *”Swarm with, produce abundantly.”
Rashi: living creatures: That there should be life in them.
a swarming: Heb. שֶׁרֶץ. Every living thing that is not much higher than the earth is שֶׁרֶץ; among the winged creatures, such as flies; among the insects, such as ants, beetles, and worms; and among the [larger] creatures, such as the weasel, the mouse, the lizard, and their like, and so [among] the fishes. [Note that in most early editions and mss., the reading is וְכָל הַדָּגִים, and all the fishes (and not וְכֵן הַדגָּים).]
Rambam: God thoughtfully willed that, from the power of continuous movement inherent in the waters, beings which have souls and which move continuously and birds that would fly over the earth across the face of the heavens should pass from potentiality into existence.
Ibn Ezra: God said effortlessly, “Let the waters use their inherent power to swarm forth multitudes of living beings composed of elemental earth, water, air, and fire. Let birds fly directly from the water over the earth, across the sky which is below the heavens.”