Assassin's Creed 4: Black Flag Walkthrough Part 15 – Sequence 3: “Sugarcane and Its Yields”

Optional Objectives:
Sabotage 2 alarm bells
Stay out of combat

Head to the North and lure the guard into the bushes with a whistle. After that you have some hunting to do. Use Eagle Vision to rule out the nearby soldiers. Look to the North, hop the next fence and head into the next Stalking Zone to wait. You will find your target before too long. Start making your way toward him carefully. When he is close, then you can easily take him down.

With the key, there is only a little left to do, namely, disable 2 alarm bells to complete the optional objectives. With the key in your possession, backtrack to the first alarm bell you came across (where the man was berated for being drunk). Duck into the nearby Stalking Zone and wait for the nearby guard to come over. Lure him in with a whistle and take him down from within the Stalking Zone. After that it should just be a short wait for the lookout guard to be facing a different way and allow you to disable the alarm.

The second alarm is to the North of the Warehouse. Head in that direction along the Eastern side of it. From that edge, duck on over to the pile of leaves. Wait for the Agent you dealt with to move away then move into the nearby Stalking Zone. Take down the nearby Alarm Guard with a whistle into the Stalking Zone. Wait for the agent to come back and do the same to him. After that disable the alarm. Even if it leads to open conflict, just do not let anyone ring the alarm bells.

With the bells disabled, head to the front of the warehouse and enter it. This will get you the loot from within there. Now you just need to head to the Northwest. There you will find a rowboat that allows for an expedient return to the Jackdaw. After that it is time to move onto the next memory.