The marginal reading is better. On the Nethinims see 1-Chronicles 9:2 note.
Ophel was the slope south of the temple (see the marginal reference "y" note); and the water-gate, a gate in the eastern wall, either for the escape of the superfluous water from the temple reservoirs, or for the introduction of water from the Kidron valley when the reservoirs were low.

Moreover the (f) Nethinims dwelt in Ophel, unto [the place] over against the water gate toward the east, and the tower that lieth out.
(f) Read (Ezra 2:43).

Moreover, the Nethinims dwelt in Ophel,.... An high tower upon the wait, in this part of it, see 2-Chronicles 27:3, these were servants to the Levites, and repaired here, where their dwellings were:
unto the place over against the water gate toward the east: and as one part of their work was to fetch water for the temple, they were here very properly situated; it led to the king's garden, the valley of Jehoshaphat, and from thence a plain way to Bethany:
and the tower that lieth out; from the wall.

the Nethinims--Not only the priests and the Levites, but the common persons that belonged to the house of God, contributed to the work. The names of those who repaired the walls of Jerusalem are commemorated because it was a work of piety and patriotism to repair the holy city. It was an instance of religion and courage to defend the true worshippers of God, that they might serve Him in quietness and safety, and, in the midst of so many enemies, go on with this work, piously confiding in the power of God to support them [BISHOP PATRICK].

Having now reached the place where the wall encloses Ophel, a remark is inserted, Nehemiah 3:26, on the dwellings of Nethinim, i.e., of the temple servants. The Nethinim dwelt in Ophel as far as (the place) before the water-gate toward the east, and the tower that standeth out. הי המּגדּל still depends upon נגד עד. The water-gate towards the east, judging from Nehemiah 12:37, lay beyond the south-eastern corner of the temple area. Bertheau, reasoning upon the view that the open space of the house of God, where Ezra spoke to the assembled people (Ezra 10:9), is identical with the open place before the water-gate mentioned Nehemiah 8:1, Nehemiah 8:3, Nehemiah 8:16, places it on the east side of the temple area, near where the golden gate (Rab er Rahme) now stands. This identity, however, cannot be proved; and even if it could, it would by no means follow that this open space lay on the east side of the temple area. And as little does it follow from Nehemiah 12:37, as we shall show when we reach this passage. היּוצא המּגדּל is said by Bertheau to have belonged perhaps to the water-gate towards the east, since, by reason of the statements contained in Nehemiah 3:31 and Nehemiah 3:32, we must not seek it so far northwards on the east side of the temple area, as to combine it with the remains of a tower projecting seven and a half feet from the line of wall at the north-east corner, and described by Robinson (Biblical Researches, p. 226). But even if the tower in question must not be identified with these remains, it by no means follows that it stood in the neighbourhood of the golden gate. Even Arnold, in his work already cited, p. 636, remarks, in opposition to Bertheau's view, that "it is evident from the whole statement that the tower standing out from the king's house, in Nehemiah 3:25, Nehemiah 3:26, and Nehemiah 3:27, is one and the same, and that Bertheau's view of our having here three separate towers can hardly be maintained," although he, as well as Bertheau, transposes both the king's house and the court of the prison to the south of the Temple area. The similar appellation of this tower as היּוצא in the three verses speaks so decidedly for its identity, that very forcible reasons must be adduced before the opposite view can be adopted. In Nehemiah 3:26 it is not a locality near the water-gate in the east which is indicted by היּוצא המּגדּל, but the western boundary of the dwellings of the Nethinim lying opposite. They dwelt, that is, upon Ophel, southwards of the temple area, on a tract of land reaching from the water-gate in the east to opposite the outstanding tower of the royal citadel in the west, i.e., from the eastern slope of the ridge of Ophel down to the Tyropoean valley.