“half of it toward the eastern sea.” The eastern sea is the Dead Sea, and this eastern flowing river is described in Ezekiel 47:1-9. This river is also mentioned in Joel 3:18. The Hebrew text reads “half of them” instead of “half of it” because in Hebrew the word “water” is always plural so the pronouns associated with it are plural, but in English we would say “water” and “it,” not “waters” and “them.” [For more on this river, see commentary on Ezek. 47:1].
“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.