A sea is defined as "a division of an ocean, or a large body of salt water, partially enclosed by land." That definition is well-suited to the salt-water Mediterranean Sea, but not as well for one of the most well-known "seas" in the world, the fresh-water Sea Of Galilee which is actually a lake (defined as a "body of fresh water surrounded by land"). The Sea of Galilee, from which The Jordan River flows southward, is however also correctly known in The Bible as a lake i.e. "While the people pressed upon Him to hear the word of God, He was standing by the Lake of Gennesaret. And He saw two boats by the lake; but the fishermen had gone out of them and were washing their nets." (Luke 5:1-2 RSV)
Literal and Symbolic Seas
"Sea" is widely used in the Bible in a literal sense:
"And God said, "Let the waters under the heavens be gathered together into one place, and let the dry land appear." And it was so. God called the dry land Earth, and the waters that were gathered together he called Seas." (Genesis 1:9-10 RSV) (see The Seven Days Of Creation and Seven Days of Creation?)
"The Lord said to Moses, "Why do you cry to me? Tell the people of Israel to go forward. Lift up your rod, and stretch out your hand over the sea and divide it, that the people of Israel may go on dry ground through the sea" (Exodus 14:15-16 RSV) (see Where Did They Cross The Sea?)
"Little by little I will drive them out from before you, until you are increased and possess the land. And I will set your bounds from the Red Sea to the sea of the Philistines [i.e. the Mediterranean Sea, the Philistine territory consisted in ancient times, just as it still should today, primarily of Gaza - see Palestine), and from the wilderness to the Euphrates; for I will deliver the inhabitants of the land into your hand, and you shall drive them out before you." (Exodus 23:30-31 RSV)
"Command the people of Israel, and say to them, When you enter The Land Of Canaan, this is the land that shall fall to you for an inheritance, the land of Canaan in its full extent, your south side shall be from the wilderness of Zin along the side of Edom, and your southern boundary shall be from the end of The Salt Sea [i.e. the Dead Sea] on the east" (Numbers 34:2-3 RSV)
"Sea," or "waters," is also sometimes used symbolically in Prophecy, primarily to represent a great multitude of people (i.e. Revelation 17:1,15):
"Daniel said, "I saw in my vision by night, and behold, the four winds of heaven were stirring up the great sea. And Four Great Beasts came up out of the sea, different from one another."
"The first was like a lion and had eagles' wings. Then as I looked its wings were plucked off, and it was lifted up from the ground and made to stand upon two feet like a man; and the mind of a man was given to it."
"And behold, another beast, a second one, like a bear. It was raised up on one side; it had three ribs in its mouth between its teeth; and it was told, 'Arise, devour much flesh.'"
"After this I looked, and lo, another, like a leopard, with four wings of a bird on its back; and the beast had four heads; and dominion was given to it."
"After this I saw in the night visions, and behold, a fourth beast, terrible and dreadful and exceedingly strong; and it had great iron teeth; it devoured and broke in pieces, and stamped the residue with its feet. It was different from all the beasts that were before it; and it had ten horns."
"I considered the horns, and behold, there came up among them another horn, a little one, before which three of the first horns were plucked up by the roots; and behold, in this horn were eyes like the eyes of a man, and a mouth speaking great things."
"As I looked, thrones were placed and one that was ancient of days took His seat; His raiment was white as snow, and the hair of His head like pure wool; His throne was fiery flames, its wheels were burning fire. A stream of fire issued and came forth from before Him; a thousand thousands served Him, and ten thousand times ten thousand stood before Him; the court sat in judgment, and the books were opened." (Daniel 7:2-10 RSV)
Fact Finder: Does God's Throne have a crystal-clear "sea of glass" before it?