This mission will be available after you find 16 Peg Leg Trinkets and bring them to the man at the port.
Optional objective: Reach the Octavius within the time limit - 1:00.
Optional objective: Do not fall into water.
Optional objective: Completed all the constraints in one playthrough.
After reaching the destination, slide down the slope and quickly reach the abandoned wreck by jumping between the ice floats.
To get onboard the ship, you need to climb the rocks marked with green dots.
From the highest of them, jump onto the wreck and go to the lower level.
In the captain's quarters you will find the fragment of map that you were looking for.
After you obtain it, you will have to run from the ship. Start off by running straight all the way and when water starts pouring inside, climb up and jump to the other side of the construction by grabbing the ropes hanging from above.
Further road leads above. After finally getting outside, the mission will end.