As the title indicates, this level is mostly water-covered. Fortunately, it's the normal water rather than the icky water you'd expect in an actual cesspool. This level will be a good deal easier than the previous one, because of the available ammo and weapons.
You begin in a surprisingly elegant room for a presumed cesspool, with two cages on either side containing Ophidians. Pass them up for now; you'll still be short on ammo. In the far corners you'll find two Yellow Doors. Continue through the center passage outside into what looks like a flooded courtyard. At the far end you'll find the Yellow Key out of reach on a pedestal. To the left and right of where you came out here you'll find doorways that are at either end of a loop passage. Although you can go either way, start with the east side. Just around the corner you'll get your Ethereal Crossbow. Further in, in a short dead-end passage to your left you'll find Wings of Wrath. Clear out the spores you find along the passage, as sometimes they're hiding items. Halfway along the passage you'll find a switch. Hit this to lower the pedestal with the Yellow Key, and when you're done exploring the passage, return to the courtyard and collect it.
Go to and open the Yellow Doors. When you enter either of them, the window just inside that looks into the loop passage will convert into a doorway. Past the east Yellow Door you'll come to a blue-and-gold tapestry on the wall. Activate this for a hidden area with items. Further in you'll find a Hellstaff in the darkened area to the left, a Green Door and a teleporter. Past the west Yellow Door you'll find another Green Door, a flooded room with cages containing Ophidians, a Dragon Claw and another teleporter. First, ready a weapon; there are Disciples of D'Sparil ahead. Take either teleporter: they both go to a square room with a pool surrounded by terraced steps. In this room you'll find the Green Key. After you collect it, take either teleporter you find in here. They go back to the teleporters that led into here, one for each.
Go to and enter either Green Door. There's a second loop route, going around behind where you began the level, and ending at the other Green Door. At the midpoint you'll find Blue-Door markers, across from which is a decorated section of wall that lowers, making a connection to just behind where you began the level. (Watch out for the enemy on top of this. Also, as an aside, the texture on the floor here is especially nice, but unfortunately is seldom seen again.) Along the west half of the route are several storage rooms with many items. Inside here, look for another blue-and-gold tapestry, and activate this wall for a door into one of the Ophidian cages near the level start. A Phoenix Rod can be found in here. On the east side is a large room with a bunch of wooden pillars (maybe these are supposed to be boxes?) and items cluttered around everywhere. In here you'll find the Blue Key. Be on guard when you go for it: as you climb up the pillars/boxes, you'll get an especially nasty Disciple-of-D'Sparil ambush along the near side of the room. Jump onto the ledge with the Map Scroll and activate the wall with the tapestry: this is a door into the other Ophidian cage. In here is a Ring of Invincibility.
Go now to the Blue-Door markers. If you try to activate the panel between them, nothing happens. Go just down the hall to the narrow wooden door, hit the switch inside here, and go back to the markers. You'll see that what looked like the Blue Door was a barrier that has now opened, revealing the actual Blue Door inside, marked with the exit symbol. In here is the exit portal, plus a Firemace, allowing you to complete your arsenal. If you haven't flown around the level yet, go to the flooded courtyard. With the Wings of Wrath you'll be able to reach those items on the pillars.