For many years, I read the words "Sea of Galilee" without thinking about it very much. It was just a name of a body of water. That changed when I finally saw this "sea". In reality, it is not a sea at all (at least the way that we think about it). It is really just a small lake. For instance, "Lake" Okeechobee in Florida is about 730 square miles in size. The "Sea" of Galilee is only 64 square miles in size. The Sea of Galilee is less than 10% of the size of Lake Okeechobee, yet one is called a sea and the other is called a lake.
So, why is it called a sea? To answer this question, you need to think about the body of water in antiquity and consider other bodies of water in the area. The other two main bodies of water in this area are the Mediterranean Sea and the Dead Sea.
In the Bible, the Mediterranean Sea is called "the great sea" (Numbers 34:6-7, Joshua 1:4, Joshua 9:1, Joshua 23:4, Ezekiel 47:10, Ezekiel 48:28, etc). The Dead Sea is called by a variety of names in the Bible. It is called the Salt Sea (Genesis 14:3, Numbers 34:3, Joshua 3:16, etc), the Sea of Arabah (Deuteronomy 3:17, Joshua 12:3, etc), and the Eastern Sea (Zechariah 14:8).
Incidentally, the Sea of Galilee is also known by a few names. It is called the Sea of Tiberias (John 6:1, 21:1) and the Lake of Gennesaret (Luke 5:1).
Did you notice anything interesting about all of these names? They are not necessarily names as they are descriptions. Think about the names again. The Salt Sea. The Great Sea. Do you see it? They are really descriptions of the bodies of water.
With that in mind look back at the name "Sea of Galilee". The Galilee is an area of land in northern Israel punctuated by wide flat valleys and numerous hills and mountains. Over any given year, it receives a lot of rain and the rich soil is very good for agriculture. There is also a body of water in the Galilee. This body of water is about 8 miles wide and 12 miles long and is the largest body of water within the Galilee. During Biblical times, the people described this body of water as a sea. It is the "sea" in Galilee, or the Sea of Galilee.
This helps explain why this relatively small lake is called a sea. Think about it.