I think they are, according to these links:
In PS3, the manholes are visible, but If I try to enter without having installed the DLC (Sewer Missions), I cannot, and states I need to get the DLC.
(For Steam the only DLC available is "The Scorchers" a totally different set of missions, but from the comments in Steam forums, it seems Steam version has the Sewers DLC builtin).
However, if you have trouble finding them all, this video in youtube has the locations of the sewers,
Adding some description:
Ark Sewers: The first is nead the Ark Area where you start the game, under a bridge an old man is camping close to the first sewer manhole.
Outrigger Sewers: Going to the Outrigger territory before taking the side path toward the Radio antenna mission, the 2nd manhole is on the side of the road.
Southern Sewers: From Wellspring, taking the left path (with a car) thru the wasted territory (like if you were to Haggar's settlement), you'll find a ramp for cars, below it, in a shadowed are is the third sewer.
Wellspring sewers: Again from the entrance of Wellspring, walk along the right (mountain borderline to the entrance) after a few meters, you'll find the fourth one.
Northern Sewer: From Wellspring again, taking the car this time the right, entering the Shroudded Territory, you'll pass under a bridge (there is a bandit tower shooting at you), then a 2nd bridge, park under that bridge, and on the left side, is the fifth manhole.
Plateau Sewer: Near the Watch Tower, you'll find a peak rock like five stories high, in a bush nearside is the sixth.
Kvasir's Sewers: It's relatively close to the Kvasir Lab (easier to connect there from the Plateau Sewer).
Subway Town Sewers: In the Subway Town Territory, drive as If you were to Gearhead Territory, but stop before shor when the cliff for Subway Town starts, on the right close to a 2 meter diameter pipe, will be a manhole.
Shipyard Sewers: Into Gearhead Territory, pick the left ramp, on the left
you'll see some green paint, very close to it, is the 8th.
Also the Rage wikia (http://rage.wikia.com/wiki/Wasteland_Sewer_Missions) list the sewers with some pictures of the places. Enjoy!