Where are the sewers?

Hint 1 of 4:

Have you found the boat house in the Lower City?

Hint 2 of 4:

The walkway to the boat house is open on one end.

Hint 3 of 4:

Notice a piece of the railing is missing.

Hint 4 of 4:

Walk toward the opening.  Arkhan will jump into the murky waters.