This quest is unlocked after completing the "Trust Is Earned" quest. To start it, head to the coordinates provided by Roberts, where he'll be waiting for you aboard his ship. Sail to the Princess and dock alongside her.
Searching for an important but unspecified treasure, Bartholomew Roberts compels Edward to sail among a massive fleet of Portuguese ships and steal one.
After returning to the Jackdaw, sail slightly to the left and pass through the narrow strait. On the other side, off to your right, you should see the ship you were meant to locate.
The flag is at the top of one of the masts of the Portuguese ship. The main issue is the two marksmen standing on the masts. Most of the sailors on deck, except for one, are looking toward your ship, so jump into the water and swim to the other side of the Portuguese vessel.
Climb up the hull of the Portuguese ship, but do not board it - instead, move over to the rigging of the mast with the flag. Start climbing when the soldier patrolling the deck is looking away.
Climb up near the platform where the marksman is standing. Wait for the moment when the marksman on the other mast will not notice your actions, then pull the soldier off the platform above you.
When the second marksman is looking away, climb to the top of the mast and cut down the flag.
Jump off the mast directly into the water, return to your ship, and take the wheel.
Quickly sail right down the middle between the Portuguese ships so their crews don't realize who you actually are, and then keep following the small ship through the narrow strait.
On the other side of the strait, you'll see the ship you're looking for, and the Jackdaw will automatically drop anchor nearby. You'll witness a small chest being unloaded from the ship.
Jump into the water, swim to shore, and hide in the tall grass. You can easily sneak through this part of the beach without killing anyone. Wait for the patrolling soldier to move off to the left, then cross the path into the tall grass on the other side.
Sticking to the tall grass, head along the rocks until you reach the pass between the rocks leading to the next beach.
After moving to the second section of the beach, it's best to wait for the guard patrolling this area and take him out when he stops near the water - no reason to bump into him or have him spot you at the worst possible moment.
Next, climb the sloping tree trunk and move directly above the soldier standing right beside the chest.
Perform an air assassination on him - the other soldiers won't notice - and open the chest, where you'll find only a few Reales.
Go around the tree, climb the ladder up to the lookout tower, and assassinate the guard standing on it.
Wait a moment for the Jackdaw to sail near the tower, then leap into the water, board the ship, and take the wheel.
Keep following the Portuguese galleon, taking care not to get too close to the ships at anchor. A bit further ahead, when the galleon docks near two watchtowers, you'll have to go ashore once again.
Jump into the water and go ashore as far to the right as possible. Hide in the patch of tall grass, then make your way to the next one next to the leaning tree trunk.
Use it to reach the tree growing beside the watchtower, and from there, get onto the tower itself.
A few jumps and the guard will be dead.
Leap onto the second tree growing next to the tower, jump down into the leaf pile beneath it, and then hide in the nearby tall grass.
Go around the large boulder from the water's edge and head through the tall grass toward the opening in the rocks - where the path leads to the area with the second tower.
Before moving on to the tall grass on the other side, make sure you don't run into the two guards patrolling this area.
Climb up another slanted tree trunk.
It won't take you all the way to the watchtower, but you can jump from it to the trees on the right and make your way to the tower from there.
Jump across the trees, leap onto the tower, and take out the guard.
Hint:
The captain, who is strolling around the ship's deck, has the key. Leap onto a nearby tree, then onto the wooden crane in the dock. To avoid being spotted by any sailors on the ship, cross onto the ship's rigging and climb up a bit. It's best to get onto the yardarm, which will let you move left and right.
Patiently wait for the captain to walk underneath you and air assassinate him. A cutscene will trigger showing your sailors taking over the ship.
To survive the naval battle, you must sink all the ships in the area. Your captured vessel is heavily armored and well-armed, so you shouldn't have any trouble. Instead of powder kegs, the ship is equipped with stern cannons. If you sail too far away, enemies might stop attacking you - if that happens, head back and look for any remaining ships.
Once every Portuguese ship is destroyed, a cutscene will trigger, ending this quest and unlocking the next one - Murder and Mayhem.