On this page of the Far Cry 6 guide you'll find a detailed description of the treasure hunt The Missing Muse, available in the Valle De Oro region.
Subregion: Cruz Del Salvador.
District: Cape Santa Maria.
To begin the treasure hunt you must read the note you'll find in a box on the blue wall.
Start the task by taking a key off the front desk.
Then focus and reach a rock which is illuminated by a small lamp. There you'll find a letter which will tell you what to do next. It's time to dive into the water and find a boat, where a chest awaits. Opening it will complete the task.