You went down the wrong sewer access point. Try another one. It should take you directly to where you want to go.
*Again - minor spoiler*
I'll make it easier on you, since I got stuck on this too. Go back to the Alley you talked to the bum in. Head to the very end of the alley and enter the sewer there.
"Welcome to the internet, pussy." - VDL "I have retard strength." - Schild