He called them not sea, but seas; because of the differing quantity and nature both of several seas, and of the rivers, and other lesser collections of waters, all which the Hebrews call seas. The separation of the waters was begun on the second day, Genesis 1:6, &c., but not perfected till this third day; therefore God's approbation of that work is not mentioned there, but here only.
Related Commentaries of Genesis 1:10
Genesis 1:10
10 And God called the dry land Earth; and the gathering together of the waters called he Seas: and God saw that it was good.