Genesis 1:21

1 the…sea monsters: The great fish in the sea, and in the words of the Aggadah (B.B. 74b), this refers to the Leviathan and its mate, for He created them male and female, and He slew the female and salted her away for the righteous in the future, for if they would propagate, the world could not exist because of them. הַתַּנִינִם is written. [I.e., the final “yud,” which denotes the plural, is missing, hence the implication that the Leviathan did not remain two, but that its number was reduced to one.]- [from Gen. Rabbah 7:4, Midrash Chaseroth V’Yetheroth, Batei Midrashoth, vol 2, p. 225].

2 living creature: a creature in which there is life.

And the Lord created the great tanins, the lev-ya-than and his yoke-fellow which are prepared for the day of consolation, and every living animal which creepeth, and which the clear waters had swarmed forth after their kind; the kinds which are clean, and the kinds which are not clean; and every fowl which flieth with wings after their kinds, the clean and the unclean. And the Lord beheld that it was good.

"that crawls" - meaning, "goes", and there are those who way that the [letter] shin is in place of the [letter] samech, and there are those who say

AND G-D CREATED THE GREAT SEA-MONSTERS. Because of the great size of these creatures, some consisting of many Persian miles — the Greeks in their books even relate that they knew some of them to be 500 Persian miles long, and our Rabbi likewise spoke of them in magnifying terms — on account of that, Scripture explicitly ascribes their creation to G-d for He brought them forth from nought from the beginning, as I have explained the expression b’riyah (creation). Similarly, Scripture does so in the case of man. on account of his exaltedness, thus informing us that man, with his mind and reason, also came forth from nought.
I wonder why it does not say “and it was so” on this day? Perhaps it would not have been possible to mention And He created after saying, “and it was so,” since it refers to the preceding., etc.) would have concluded with the expression and it was so, it would have implied the creation of the sea-monster, since they also swarm in the waters. How then could it say in the following verse, And G-d created the sea-monster, as if it were another act of creation? To say, and it was so after vayibra (And He created) is also not possible for this would be redundant. Hence on the fifth day, the expression and it was so does not appear in Scripture.
Our Rabbis have said that the great sea-monsters are the Leviathan and its mate which He created male and female. He then slew the female and preserved it in salt for the benefit of the righteous in the hereafter. It is possible that on account of this it would not have been appropriate to say concerning their creation, “and it was so,” since they did not continue to exist [in the form in which they were created].

התנינים הגדולים, even though, in the previous verse, the Torah reports on the directive for the waters to produce multiple reproductions of each species which has its habitat in the waters, and we could have thought that this did not include gigantic creatures such as whales, the fact is they (it) too had been produced by the waters at the same time. The species described as Leviathan, Nachash beriach and Leviathan beriach in Isaiah 27,1 and the Leviathan described in Job 40,25 are what our verse here refers to.

ויברא אלוקים את התנינים, the evolutionary powers of the waters did not suffice for this task. They could not produce the original sea-monsters without having been provided for this with seed by the Creator.

ויברא אלוקים את התנינים, we do not find the expression ויהי כן in this paragraph. The reason that the Torah had to introduce this paragraph with the words ויברא אלוקים, “G’d created,” as distinct from “the waters brought forth,” or something similar, is that the waters were not capable of producing creatures of such dimensions and capabilities without additional input by G’d Himself. Both G’d and the waters combined to produce these monsters, hence their creation is described by the word ויברא.

אשר שרצו המים, you should know that in parts of the oceans which are far distant from the dry land there are no fish, seeing the waters by themselves are unable to sustain their lives by supplying the fish with additional food such as plants which grow on land or vegetation on rocks which are close to land, or refuse left by man as food for the fish by the passing ships. Fishermen do not explore the areas we have mentioned, as it is impossible for their boats to enter such waters without risking their lives. Fishermen are alerted to the fact that they approach such areas by the absence of fish in the regions close to the ones described. The oceans are constantly stormy in such regions.