The sealed sewers are accessed from the central sewers. A key called Luke's find must be obtained to enter, it is found on "Blind" Luke in a room in the central sewers or "Sweet" Jill in a room in the east central sewers.
The door to the sealed sewers has water halfway up the door from one side, but not the other. However, it opens from both sides without water spilling through, which should be impossible.
The sealed sewers appear only in Fallout: New Vegas.
Playstation 3 Xbox 360 It is possible for companions to get stuck in the entrance of the sealed sewer, rendering them incapable of moving from their location. [verified]