Jamieson, Fausset and Brown on Genesis 1:9

Ge 1:9-13. Third Day.

9. let the waters under the heaven be gathered together unto one place--The world was to be rendered a terraqueous globe, and this was effected by a volcanic convulsion on its surface, the upheaving of some parts, the sinking of others, and the formation of vast hollows, into which the waters impetuously rushed, as is graphically described (Psa 104:6-9) [Hitchcock]. Thus a large part of the earth was left "dry land," and thus were formed oceans, seas, lakes, and rivers which, though each having its own bed, or channel, are all connected with the sea (Job 38:10; Ec 1:7).