Once you have acquired the LOCK PICK, you can return to earlier levels to find and open treasure chests, The one in Cozumel is in the Temple of the Moon (Pyramid) area.
To reach it, start by using any base camp to fast travel to the Tidal Caverns Base Camp. head toward the edge of the chasm.
You won't need to solve the puzzle again, but you can jump across the bell platforms to reach the ledges you traversed earlier. Start by jumping to the first bell, then the second.
The third bell now has a mine cart on top of it, but you can still grab the edge of the platform, climb onto the cart, then jump to the ledge.
From there, climb the stairs...
...up and around to the fourth bell. (If you have not revisited this area in a while, the gold on the platform at the top of the stairs should have respawned.)
Jump to the fourth and fifth bells...
...then to the ladder on the ledge in the southeast corner. If you missed the document, it's sitting here on a little shelf. Continuing to the treasure chest, follow this ledge to the far end.
Use a rope arrow to anchor a line so you can climb up to the ledge on the south side/ (If you missed mural #2 earlier, it's just ahead on the right.) Continuing to the treasure chest, turn around to face the rope line you just climbed.
This is a good time for a reminder about setting waypoints on the in-game map. If you open the map, move your cursor over the treasure chest (assuming you already found the explorer backpack that revealed it), and press the button indicated on-screen for Toggle Waypoint, this will mark the chest with a blue circle.
It will also be marked in the game world when you activate Survival Instinct.
To get over there, shoot another rope arrow into the rope-wrapped beam above the post you used to anchor the first rope line. The new line will replace the old one.
Climb across and drop down in the doorway.
You'll find some gold ore lying on the ground (assuming you didn't pick it up earlier or enough time has passed for it to respawn).
The treasure chest (1/1) at the far end of that ledge. Stand near the chest and hold Interact to open it with the LOCKPICK. The Gold Mask inside boosts Lara's fluency in the Yucatec dialect. It doesn't count as an "official" relic, but it is automatically added to your Artifacts collection under the Pantheon of Gods 2 Story.
To get back, move out to the corner of the ledge on the left side of the chest and jump down onto the fifth bell, then the fourth, then the ledge below. Head down the steps to the edge overlooking the lower set of bells.
Hop down onto the third bell (with the mine cart on top), then from there to the second bell, then back to the first.
Finally, jump from the first bell to the climbable wall, and climb up to the Tidal Caverns Base Camp. Either continue exploring or use Fast Travel to return to wherever you left off.