“half of them toward the eastern sea.” The eastern sea is the Dead Sea, and this eastern flowing river is described in Ezekiel 47:1-9.
“the western sea.” The western sea is the Mediterranean Sea.
“in summer and in winter.” This statement was very comforting to the people in Israel in biblical times because most of the rivers (actually “streams”) in Israel were not perennial, but dried up in the summer months. The rain in Israel usually fell from late October until mid-to-late April, and then it was without rain from May until October, and most of the streams dried up in those six rainless months. Here in Zechariah God promised that the river from His Temple would flow both east and west all year long.