Thanks for your question.
The significance is that means what it says. A sea is a large body of water. It is the Greek word Thalassa and, although it would normally be used of a body of salt water like the Mediterranean Sea, it was also used to describe any large body of water such as the Sea of Galilee. In Mark's Gospel, this word, Thalassa, appears 15 times and every time except 2, it particularly refers to the Sea of Galilee. In Mark 9:42 and 11:23 it refers to any deep, large body of water but not specifically Galilee.