There is one sewer pipe that leads to the Verdistis sewers, and another that leads back near the Ars Magicana market square entrance of the Rivertown sewers.
You do not have to worry about getting any quests before exploring the Verdistis sewers.
Make sure your spiritual resistance is at least 10 or 12, though, or save before opening chests. There are 2 trapped chests that can permanently turn you into a frog or rabbit, requiring you to reload your last save. During development of the game Polymorph was made permanent and all the traps were suppose to have been removed, but 2 in the Verdistis sewers were missed.
