New Main Quest: Escape the sewers to meet a new ally
If you nowa want to , make any alterations in your equipment, you should press and hold down the middle mouse button (I recommend pairing up your blade with your crossbow with sleeping bolts). Otherwise you should use the key from the chest to open the nearby gate. You'd better avoid the new trap by following the game's suggestion, which is by running (the "SHIFT" key) and sliding (by pressing the "C" key - the above screenshot). After you do that, collect the incendiary bolt from the launcher to the right.
Remain in the location where you avoided the last one of the traps. Look around for food and a note (Jelly's share). Read the note and explore the area to find digits 451 painted behind the empty bottles. Now approach the safe shown in the above screenshot and enter the combination , which will lead to opening the safe. Inside you will find gold and Sokolov's health potion.
You may now go on (easy climb). After you reach the next area, you I recommend that you mind your steps because you are nearing the only area in the sewers with guards in it. Explore the area for food, health and Sokolov's health potion. Decide now whether you want to jump into the main hole or into the side hole to near yourself to the edge (the above screenshot).
Traditionally now, you should wait for the guards to disperse. All in all, there should be only one guard left below and you may do one of three thins here. If you don't mind eliminating your opponents, jump down onto the guard and perform execution (left mouse button right before landing). If you want to put him to sleep, jump down right behind him (the above screenshot) and press and hold down the left "CTRL" key. If you choose to avoid him, wait for him to go right and quickly start exploring further part of the sewers.
It's quite a pleasing information that you can easily avoid encountering all of the remaining enemies patrolling the sewers. You can pass overhead on pipes above (the above screenshot) or use the chain of underwater tunnels. In the case of the former, you need you would have to turn left immediately after reaching the next area. If you however dream of taking a diving class, just remember to resurface regularly underneath metal footbridges, which will ensure not being noticed.
You can equally well eliminate the three guards around, but due to dim lighting of the area might this prove burdensome. I recommend that you wait until two of the guards position themselves at the two balustrades shown in the above screenshot. After you dispose of them, deal with the remaining one. Because he changes his location often, it is best to swim up to him.
If you've eliminated all of the guards, you should devote a few moments to finding gold, food and Sokolov's health potion (on the side balcony connected with rocks that you first need to swim up to). Otherwise you should immediately go towards the passage leading up to the next part of the sewers (the above screenshot).
Go down the quite linear path. On your way you should take a look into one of the trash containers, where you will find food and gold. You need to reach the Dunwall sewer exit shown in the above screenshot.
New Main Quest: Talk to Samuel the Boatman
Approach Samuel and initiate a conversation with him. At the end of the meeting you need to decide if you want to leave the area right away (the upper-right dialogue option) or you want to remain here (the middle dialogue option). In the latter case, you might wish to explore the nearby water reservoir in search of additional loot. I recommend against it, though. There are a lot of piranhas in these waters and you'd have to kill them from the surface or lose a lot of health points. Leaving with Samuel means completing this mission.