The Great Sea is the name of the body of water west (toward the sunset) of Canaan and Philistia. It's northern shores were populated by the descendants of Jephthah, the oldest son of Noah in the days of Peleg and the dispersion from Babel. It the south, the great Egyptian culture flourished.
First encountered by Abraham on his way to Egypt, the Great Sea was a place to avoid. Ever since the days of Noah the open sea had left a vivid image on the collective mind of his descendants.
The Great Sea (Heb: הַיָּ֥ם הַגָּדֹ֖ול, "the Sea, the Great") was known as "great" in relation to all other bodies of water known to the writers of the Bible. It is also called the Hinder (Western) Sea (הַיָּם הָאַחֲרוֹן) being behind those accustomed to orienting themselves toward the east. It is once called "the Sea of the Philistines" (יָם פְּלִשְׁתִּים).
Though a vast body of water, the Great Sea is dwarfed by its neighbors to the west and the south, its central location may be a remnant of the "waters in one place" of the second day of creation. It was into its depths that much of the water of the Great Flood receded.
Years later, Abraham sojourned in Egypt, he would have seen the vast ocean sandy beaches reaching for miles. It would then be more understandable when God spoke of the "sand of the seashore." When the tribes moved to Egypt years later, they were located near the sea. Tradition has it that Zerah, Judah's son, was an explorer that took to the sea. It makes sense, but nothing in the Bible backs it up.
Though some of the tribes were given lands that bordered the Great Sea, it was not until the time of David that an alliance with the king of Tyre made the Great Sea necessary for trade. From the forests in the north, King Hiram was able to ship logs down to a port city to be processed before being moved to Jerusalem to prepare for the temple that Solomon would have built.