The entrance to the sewer is located near the Watch Tower. The sewer hatch is partially obscured by some weeds and a large rock. Look for the Vehicle Jump "Dedicated", marked by the hovering Authority drone nearby. The drone is between two large stone pillars. The pillar furthest from Watch Tower, on the western side, has the sewer hatch hidden in the bush. If the drone is still there, stand underneath it and walk southeast, you should run into the sewer entrance.