On this page of the Far Cry 6 guide, you will find a detailed description of The Long Drop Treasure Hunt available in the El Este region.
Subregion: La Joya.
District: Catalina Ridge.
Once the mission begins, immediately head to the other side of the destroyed bridge. You can easily jump over the hole when you use a climbing rope.
Get to the building where you can control the surrounding devices. Your job is to hold back the water so you can easily get to the corpse lying by the dam.
Start by using the button on the yellow object.
Then interact with the orange button, on the metal crates.
As you continue onward, you will come across a valve, located on a blue pipe. This is almost the final piece.
Finally, you must enter the smaller room and use the button on the blue object. This will shut off the flow of water and allow you to get the key safely.
This is where you can slide down the rope or just jump off. Take the key from the corpse and go to the locked room.
In the small building where you started the mission, you will find a weapon crate. Opening it will complete this task.