"Use 3 Sleep Darts on crocodiles" is an optional mission objective in Sequence 6 / Memory 3: Siege of Charles-towne
Walkthrough[edit]
Once you begin the objective of Tail the rowboat you will see this optional objective become active. Just to the North of that location, before you enter the ruined hull of the ship in front of you, you will find the first crocodile. Quickly tag it with a Sleep Dart. This is also an ideal Crocodile to skin as well.
The second crocodile is found just after you exit the first ruined hull. Look down into the water below. You will see it heading after the boat.
The third crocodile is found just to the North of the second one. You need to be quick to tag it with a sleep dart. This will make the shortest work of the optional objective.
