On this page of the Far Cry 6 guide, you will find a detailed description of the Sword-Crossed Lovers treasure hunt in the Valle De Oro region.
District: Tormentoso Bay.
Start the task by reading the note. Next, you must break the boards that block the entrance to the tourist attraction. This is not a difficult task, all you have to do is hit them with a machete.
Keep walking forward until you reach the ship. Your task is to hang the rope, go all the way down and fire the cannon. This will open the passage further.
It's time to climb up and get into the room that was blocked off earlier.
Collecting the chest will complete this task.