Or HaChaim on Genesis 1:8/אור החיים על בראשית א׳:ח׳

1And He called: Here the verse informs us these are the [same heavens] that were mentioned in the verse, “In the beginning etc.” And [the reason] that the first verse ascribes the name, 'heavens' to them is to say that there is [here] a creation that will be called heavens in the future; but really God did not call it this name until the second day.

2And according to the words of our rabbis, who said (Bereishit Rabbah 4:7) that the understanding of shamayim (heavens) is sa mayim (support the waters), the intention of [this] verse is according to the following way: “And God called to the heavens and said” to it, support the waters, so that there will be [space] to [make] room for the creatures, so that the creatures will be between the heavens and the earth; and without this, there [was] no place for the dwellers of the earth, until He called upon the heavens to support the waters that were laying upon [the earth]. And he told the lower waters to gather to one place on the third day and the dry land appeared with this, [such that] there was [now] a place for the dwellers of the earth. And this is not a contradiction to the words of our Rabbis, that say that the upper waters are held up miraculously; since even if it is the case that the heavens support them, nonetheless with regards to the set up nature of the skies and their order, they felt that they were supported by a miracle, as we have written on the verse, “And he made.”