This page of the Far Cry 6 guide describes a treasure hunt Crocodile Tears available in Madrugada.
Subregion: Aguas Lindas.
District: Ardiente Cay.
To start the treasure hunt you must read the note found in a box, right next to the pink building.
There are a lot of crocodiles in this area. You need to get rid of them so that they don't attack you while you explore.
To proceed you must find the largest enemy. He has the key you need.
Get the key and head to the room it unlocks. You can do so be destroying two padlocks blocking your path. Then climb the stairs and make a long jump on the rope.
You can also ignore the padlocks and walk out the back. From there, you'll also reach a place you can attach a rope to. Pull yourself up and start swinging, trying to reach the door.
You'll see the crate once you get through the door. Open it to complete the quest.