Another interpretation: because the work of the waters was not completed. Therefore it wrote "that it was good" twice on the third day, one for the work of the waters and one for the work of the day. A certain noblewoman asked Rabbi Yose: Why is "that it was good" not written on the second day? He said to her: Even so, He went back and included them all at the end, as it is said, "And God saw all that He had made, and behold, it was very good" (Genesis 1:31). She said to him: A parable. Six people come to you, and you give each one a portion, but to one you do not give; then you go back and give all of them a portion. It turns out each one has a portion and a sixth, and one has only a sixth. He turned back and said to her: Because the work of the waters was not completed, and so forth.
Why the Second Day Lacks the Words It Was Good
Yalkut Shimoni on Torah 5:7
דָּבָר אַחֵר, שֶׁלֹּא נִגְמְרָה מְלֶאכֶת הַמַּיִם לְפִיכָךְ כָּתַב בַּשְּׁלִישִׁי שְׁנֵי פְּעָמִים כִּי טוֹב, אֶחָד לִמְלֶאכֶת הַמַּיִם וְאֶחָד לִמְלֶאכֶת הַיּוֹם. מַטְרוֹנָא אַחַת שָׁאֲלָה אֶת רַבִּי יוֹסֵי, מִפְּנֵי מָה אֵין כְּתִיב בַּשֵּׁנִי כִּי טוֹב. אָמַר לָהּ, אַף עַל פִּי כֵן חָזַר וּכְלָלָן כֻּלָּן בַּסּוֹף, שֶׁנֶּאֱמַר (להלן פסוק לא) "וַיַּרְא אֱלֹהִים אֶת כָּל אֲשֶׁר עָשָׂה וְהִנֵּה טוֹב". אָמְרָה לוֹ, מָשָׁל שִׁשָּׁה בְּנֵי אָדָם בָּאִין אֶצְלְךָ וְאַתָּה נוֹתֵן לְכָל אֶחָד וְאֶחָד מָנָה וּלְאֶחָד אֵין אַתָּה נוֹתֵן, וְאַתָּה חוֹזֵר וְנוֹתֵן לְכֻלָּן מָנָה, נִמְצָא בְּיַד כָּל אֶחָד מָנָה וּשְׁתוּת וּבְיַד אֶחָד שְׁתוּת חָזַר וְאָמַר לָהּ, לְפִי שֶׁלֹּא נִגְמְרָה מְלֶאכֶת הַמַּיִם וְכוּ'.