Why was the Sea of Galilee called so? We see the mention of a freshwater lake of 167 km square in Israel called as the Sea of Galilee or the Sea of Tiberias at Mtt 4:18, Mtt 15:29, Mk 1:16, Mk 7:31 and Jn 16:1. I wish to know the logic behind calling a lake a sea.
This is not an unusual usage, nor one that needs a specifically religious explanation. "The Sea of Galilee" is the standard English translation of the original Greek θάλασσαν τῆς Γαλιλαίας ("thalassan tes Galilaias"); that is, "the θάλασσα of Galilee"). One sees this usage, for example, in Matthew 4:18. 

Liddell and Short, a commonly used Greek-English lexicon, defines θάλασσα as "the sea", using primarily references to the Mediterranean. 

Merriam-Webster gives a usage of "sea" to mean


  a great body of salt water that covers much of the earth


that is, a synonym of "ocean". However, it also describes "sea" as used to mean


  an inland body of water—used especially for names of such bodies


It is in this sense that the body of water you mention is called "the Sea of Galilee".