I'm writing this walkthrough from the perspective of having entered the sewers from the northeast corner. Be aware that there are roaches and Poison Fungus through this entire level. I have opted not to mark them all on the map.
When you first enter the only way to go is south, but watch for the passage leading to the east, which leads to small room. In the northern corner of this room is a secret passage which leads to a few halfway decent items.
Go back to the main passage and continue following it south. When you get to the point where it continues south or branches west, continue south. There's a section of wall in the southeast corner of this room that you can crush with Move Mountains or a Stoneshatter Crystal. There are also a few dead bodies in this area, which have some fairly weak equipment on them.
Now go back to the branch and head west. When given a choice between south and west, continue going west, since the southern branch just leads to the other exit of the sewers. Notice at this branch the small island across from you with a really big roach on it.
After crossing two bridges you should notice that there are some cracks in the southern wall here. Use Move Mountains or a Stoneshatter Crystal to reveal the body of a mage. There's a spellbook on this body which teaches Bolt of Fire Lv.3.
Now continue along the edge of this island and towards the northwest corner. In the northwest corner is a Wand of Lightning on the ground, and a boat. Board the boat and head over to the island with the big roach. Once you've killed the roach you can pick up a few potion making herbs.
Get back in your boat and head up to the north, across from the entrance. If you've been given the quest to find Grunders ring, and have at least one point in Luck you'll find his ring here.
Finally, head over to the far west and go under the first bridge you crossed upon entering. This leads to where the trash of Shayder is thrown down. You'll find a Wand of Fire down here.
Now head back up to Shayder and we'll go check out the Anama Temple.