The “great sea which divideth the lands” is either the Pacific or the Atlantic, but this description contains no identifying details. Moroni is picking up an interrupted narrative (“and now I proceed with my record…”), that had begun in our verse 8. Verse 7 communicates that “the Lord would not suffer that they should stop beyond the sea in the wilderness, but he would that they should come forth even unto the land of promise.” Reading from this verse past Moroni’s discussion of the promised land, we learn that the Jaredites did not stop “beyond the sea” until they reached a camp where they pitched tents (v. 13) and stayed for four years (v. 14).
I interpret this narrative text as saying that they traversed the Mediterranean (the “sea”) and stopped at the Atlantic coast. They lingered past a reasonable period for provisioning, despite Yahweh’s command that they not “stop beyond the sea in the wilderness,” prompting his chastisement of the brother of Jared (v. 14). There is no indication of travel during those four years.
Culture: They name the area Moriancumer. Knowing that this word was part of the brother of Jared’s name (see commentary accompanying Ether 1:34), it is easy to deduce that the area was named for him. Probably Jared was reserving his own name for the permanent colony in the land of promise.