Optional Missions:
- Loot 3 treasure chests
- Avoid being attacked by a shark
Head to Nassau to begin this particular mission. After that, join Ben and Blackbeard on a walk. After the conversation, you'll have to buy a diving bell for your ship next. You should have enough funds at this point so find the nearest harbourmaster and purchase a diving bell for 5000 reales. Return to your ship and sail to the sea. Head to the wreck of San Ignacio to the west and meet up with Blackbeard.
Head to the wreckage, then dive down. The Diving Bell will allow you to replenish your air supply as many times as you need. There are also scattered air barrels dropped by your crew where you can enter to replenish your air and to escape from sharks. These are used once only. In caverns, there are natural air pockets where you can replenish your air supply as many times as you need.
The map of the underwater wreck / area is already complete and all important items are already listed there. You just need to locate all chests and find all Animus Fragments. For the first area, make sure you collect all chests and a single animus fragment before opening the hatch of the wrecked ship and getting the medicine since the wreckage where the medicine is located will collapse, preventing you from going back where you came from.
After getting the medicine, the wreck will collapse. You have no choice but to swim through the underwater tunnel. Use the air pocket there to replenish your air as you go through the tunnel. As you move on, you'll caught up in a violent water current. Wave your arms to control your body and prevent yourself from hitting the rock pillars. Keep at it until you reach the next part of the tunnel.
As soon as you exit the tunnel, you'll encounter the dreadful predators of the sea; great whites. You have to hide in the seaweed or Shipwrecks to avoid getting attacked. Take note that unless you're near them, they won't attack immediately; you'll hear an audio cue indicating that you've been spotted by a shark (or sharks). Use that time to immediately find seaweeds, wreckage or air barrels where you can hide. Moving on, use the air barrel nearby if you need to replenish your air and continue collecting the remaining collectibles in the area. There are three sharks roaming around and you'll have close encounters with them often since the area isn't that large either. Once you've explored the area completely, carefully return to the diving bell and resurface to complete the mission.