Or HaChaim on Genesis 1:21/אור החיים על בראשית א׳:כ״א

1And God created, etc.: The explanation is that God added power to the two parts of creation that were created from the water. And that which it repeated to say, "and all the living souls, etc." and that the waters did not suffice to bring them out - which we did not find with what was done by the ground in the bringing forth of grasses and herbs, [whereby] the proclamation of God was sufficient for it - you should know that neither a grower nor a creator can create more that what is in the potential of its [own] growth, since how can a person give what he doesn't have; and behold, the water and the dirt lack living spirit, and from where would they have spirit to give to those they create. And even though the earth and the waters have a very fine life force, even if they give from it, they would not [be able] to give more than what is in them, and this would be insufficient for the fish and the birds; so therefore, their power was not enough and God had to create a living spirit and this is [what is meant] when it states, "And He created, etc. all the souls, etc." And if in His command to the waters, He said, "Let the waters swarm with swarms of living souls," the explanation is [that] the swarms be set up to receive living souls. Since you need to know that life force [requires] a setup and that is the intermediary between it and between matter, and the ability to create [such a set up] exists in the waters, since, as we have said, they have a fine spiritual power.

2And in the great sea monsters, God made a change in their creation - as we have said - and He didn't state in [a more] general [fashion], "And He created the souls of the sea monsters and all the souls, etc.;" by which the verse revealed that they are different from everything [else] in the waters; since besides that God implanted a soul within them, also in the power of their creation itself, God created an extra power within them; since there is no power within the waters to make it, as we have said. And this is the amazing fish, it is the leviathan, and its matters are explained in [the] words of [the Sages], of blessed memory (Bava Batra 74b). And the reason it did not state about this creation, "and it was so," is that there is no place for it, since it is not needed to say "and it was so" except when only the proclamation of God is stated; [there] the Torah tells "and it was so," since He [had] said, "let there be;" which is not the case when it states, "And He created" - proclamation is included in it, execution is included in it.