"A smuggler chief has crossed the line by selling his stolen good to the Templars and killing innocent men and women in the process. Take his life so that he may not harm others."
Getting through the underwater maze takes some doing: From the Diving Bell start, swim to the opening to the South of there. Travel through the submerged mine tunnel until you find a ladder. Follow it up. Pause at the junction ahead to let the shark swim on ahead of you. This shark swims a counter clockwise circuit through the area. You need to press into its territory to refill your breath and to open the chest int he same area. As soon as it passes by then take the tunnel to the left. Down this way you will find an air pocket. Return to the corridor with a fresh lungful of air. Wait for the shark to go by again Turn left and swim to the South until you reach another ladder. Swim on upwards. When you get to the top you will find a chest in the dead end to the east. Make a u-turn after that and head to the Air Pocket to the Southwest. Continue through to the South until you reach a junction. Turn left there to find an Animus Fragment. Once again, make a U-turn and swim West until you reach a ladder. Lunge up to the surface and climb upwards. There is one more swim to get to the Smugglers but it is very straight forward.
Dive into the water and go right at the first intersection. Follow this corridor a while. Turn to the left when you can, twice. be sure to use the lunge bar to speed up Edward's progress after the second left. Head to the end of the corridor and climb on up.
When you make it through to the cave inside the dive, watch the central area of the cave. Wait for the patrol to go on through. After they have you will see your target at a distance. Climb up onto the the nearby dock and perform a ledge assassination on the passing by pirate once the patrol has gone by. Climb up and kill the person attacking the prisoner to the left after that. Now go down the dock toward the central area. Fire a Berserk Dart at the Captain to finish him off. #YOLOSWAG MLG