What did the command "Let the water gather... and the dry land be seen" entail?
Rashi: Until now, the (lower) water covered the entire earth, but now, Hashem ordered it to gather in one area, known as the ocean, so that the earth now became visible. 1
Ramban #1: Initially, the earth and the water were mixed to form a muddy substance called 'Tehom' (the depth). Hashem now issued two commands: a. that the water should all gather into one location (to form the seven seas); b. That the earth should rise above the water, for the people to inhabit it. 2
Ramban #2: Alternatively, the second statement refers to the earth taking on the shape of a ball, some of it visible, the majority, submerged - though this does not affect the dual command of Hashem.
Seforno: Hashem commanded the water to gather in one place and to remain there. This meant that it was not to go back and cover the earth, as it ought to have done naturally and as it had done originally.
Oznayim la'Torah: It entailed the water collecting in one expanse and leaving the rest of the world complely dry - miraculously - seeing as it would take the world two months to dry following the Mabul. 3
Hadar Zekenim asks, once the water gathers, obviously the dry land will be seen! He answers that the latter words allude to Keri'as Yam Suf.
Despite the heavier nature of the earth, in which case the water ought to have covered the earth on all sides. The Seforno explains that in fact, the Torah adds "and it was so," to stress that it remained so, even though it defied the laws of nature (See answer 3).
This also explains why the Torah writes in 2:5, that the vegetation had not grown before Adam arrived on the scene because it hd not yet rained. Now if the world had not dried completely after the water collected in the sea, the wet surface would have sufficed to water it. See Oznayim la'Torah.
