Were there Four Seas or One Great Sea?

Theorists today are debating whether the Book of Mormon describes four seas, or five seas, and promotes the idea that these seas are not connected, but separate bodies of water, thus giving them the ability of naming places like the Great Lakes as the four seas mentioned in Helaman 3:8. In this case, the Great Deep is seen as a separate sea and therefore they can also lay claim to lakes being used as seas instead of oceans.
As one theorist claims, “It's a guessing game. We don't know what they called the sea.”
However, the scriptural record is quite specific about this, and certainly the scriptural record does not lend itself to a guessing game. In their claims, they have the four seas as North, South, East and West. Their fifth sea is the Great Deep, mentioned three times in Ether as spoken by the Lord to the Brother of Jared (Ether 2:25; 7:27; 8:9)
Your comment regarding “We only know there was 4 seas or 5 if you count the great deep,” is not an accurate statement. As an example we do not know there were four separate seas around the Land of Promise as North American theorists claim. What we know is that there were four directional names given to the sea or seas around the Land of Promise—one to the east, another to the west, another to the north and another to the south.

Left: Four separate lakes (actually there are five, but theorists ignore the fifth one, which happens to be the largest; Right, one surrounding ocean, named separately according to cardinal direction. Note that the left map has the South Sea to the north of Bountiful, to the east of Zarahemlah, and to the north of the majority of the lands of Bountiful and Zarahemla, and the Land of Nephi—all contrary to the scriptural record (Alma 22:27-34)

The truth of the matter is that we do not know if this means there was one body of water around the Land of Promise given names of the direction portions lay to the four cardinal points.
    In other words, was the East Sea part of a larger sea, such as a large ocean like the Pacific, or was it a small sea like a lake.
    Regarding the names of the seas and how the Hebrews named them, we see that they used directions to name their seas—and those directions were the farthest area away from them in the cardinal directions: North, South, East and West.
    Based on Jacob’s description of sailing across the ocean to an island where Lehi landed, keeping in mind that he was on Nephi’s ship during the voyage and landing, we find that the sea surrounded the island and that the Nephite names referred direction, such as the sea to the north; the sea to the south, the sea to the east, and the sea to the west.
    In the Levant, the Mediterranean Sea was called the "Hinder Sea" by Israelites because of its location on the west coast of the Holy Land. It was called the “Great Sea,” because of its size and domination the entire western horizon. Israelites also called it the "Sea of the Philistines” from the people inhabiting a large portion of its shores to the west of Israel. In modern Hebrew it is called HaYam HaTikhon meaning “the Middle Sea” (Andrew P. Vella, “Mediterranean Maltas,” Hyhpen, vol.4, no.5, 1985, London, pp169-172). In Classic Persian was also called “The Western Sea” (Daryāy-e Šām).
    On the other side of Israel, lies the Dead Sea, called by the ancient Israelites (ha-Yām ha-kadmoni הים הקדמוני) the “Eastern Sea.” In Arabic it was called Bahr al-Maghrib, “the Sea of the West.” In addition, the Romans called it the Mediterranean, meaning the “Inland Sea.”
    In fact, around 520 BC, the Hebrew prophet Zechariah states: “"living waters will go out from Jerusalem, half of them to the eastern sea and half to the western Sea" (Zechariah 14:8).
    It might also be of interest to know that in the Levant in classical antiquity, cultures used colors to refer to the cardinal points: black referred to the north (explaining the name Black Sea), yellow or blue to east, red to south (the Red Sea), and white to west.
    Thus, we see the Mediterranean Sea called by the Greeks Άspri Thálassa, the Bulgarians Byalo More, the Turkish Akdeniz, and the Arab al-Mutawassiṭ, all meaning the “White Sea” or “West Sea” (Universal Linguist: Johann Knoblock, “Sprache und Religion,” vol.1 (in Carl Winter Universityh Press, 1979,, 18; Rüdiger Schmit (1989). "Black Sea". Black:  Encyclopaedia Iranica, vol.4, Fasc.3, pp310-313).
    Consequently, we know that to the Israelites, and many peoples of the time, seas were often named by the direction they were from the naming source. In Israel there was a Sea East and a Sea West.

The Sea That Divides the Land is an extension of the Sea West

In addition to the four directional seas named, there is another area said to be by the narrow neck of land, by the place where the sea divides the land (Ether 10:20). Now this sea that divides the land has to be evaluated in light of the two seas that flank the narrow neck of land. As Mormon stated: “by the narrow pass which led by the sea into the land northward, yea, by the sea, on the west and on the east. Thus, the narrow neck is flanked by two seas, the Sea  East and the Sea West.
    This means the Sea that Divided the Land at the narrow neck, had to be connected to one of those two seas—that is, an extension, such as a large bay or gulf that could be seen as a separate water area of water, but still connected to or part of the sea. Thus, we cannot call it a separate sea, only that this portion of the sea cut in to divide the land.
     As for “the great deep,” this was not a separate sea, but the path over which Nephi’s ship sailed. Jacob said it quite clearly when he told the Nephites that “the Lord has made the sea our path, and we are upon an isle of the sea” (2 Nephi 10:20). That is, the Lord led them across the great deep (what the ocean is called) to an island in the midst of the sea.
    To understand the usage of the term “great deep,” we need to look at the meaning of the term in ancient Hebrew
    The Hebrew word תְּהוֹם “Tehom” literally means “Deep” or “Abyss,” and means the “Great Deep." This refers to the primordial water of creation in the Bible, and is first mentioned in Genesis 1:2 where it is translated as “deep.” It was written in the Greek as “Pléroma” or “Bythós,” meaning deep. Tehrom is a cognate of the Akkadian word “tamtu” and Ugaritic “t-h-m,” both meaning “deep.” All of these are associated in their own languages as the “deep ocean,” or “great deep.” Earlier, these words were equated to the Sumerian Tismat, and in modern Arabic, to Tihamah. Thus, "Tehom," used as the technical expression for the primæval ocean, and more modern as the Deep Ocean, or what is called the Blue Water of our day, which means the “open ocean” or “deep seas.”
    The word comes from two Hebrew words, , rab (an adjective meaning, much, many, great) and תהום, tĕhōm (a noun meaning, water making noise and translated as deep, depths, deep places, abyss, the deep, sea). Tĕhōm is derived from the root verb הום, hūm meaning to make an uproar or agitate greatl^y. In the Hebrew language, the phrase תהום רבה, tĕhōm râbâh became a stereotyped compound noun and therefore always used without the definite article, thus there was not this or that great deep but rather an all-encompassing great deep, Franz Delitzsch translated it as unfathomable ocean (Gerhard F. Hasel, The Fountains of the Great deep, Origins, vol.1,no.2, 1974, pp67-72); Alfred Rehwinkel, The Flood, in the Light of the Bible, Geology and Archaeology, Concordia Publishing House, Saint Louis, MO. 1952, p100;Roy Allen, Fountains of the Great Deep, CRSQ, vol.33, no.1, 1996, p19).

The five world oceans are all connected together, which prompted Lehi to say as he looked out over the Indian Ocean, that it was “many waters,” which he understood either by instinct or revelation

Thus, the Pacific Ocean, Atlantic Ocean, Indian Ocean, etc., are one overall ocean and referred to as the Great Deep, or the deep water, as opposed to the shores. The word “Blue Water,” used today for the great depths of the oceans, was first introduced in 1582 and has come to mean to most mariners as the deeper parts of the ocean, thus, crossing the Great Deep would be crossing from one shore to another by going across an open ocean, as the Jaredites did. And, using the same phrase, Nephi tells his brothers how their lives were spared crossing the ocean (2 Nephi 4:20).
    Thus, we can conclude there were four seas in the scriptural record surrounding the Land of Promise. What the large area of waters the Jaredites called Ripliancum is unknown, though we do know it was in the north, and would either mean it was the sea to the north, or a large lake in the Land Northward.
    Simply Put, if we rely on the scriptural record and not try to make something out of it not intended, we have a better chance of understanding it and what was written in both content and descriptions.