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 to 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 did originally.
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 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).