This page of the Far Cry 6 game guide contains a walkthrough of the "A Rising Tide" Treasure Hunt available in El Este.
Subregion: Sierra Perdida.
District: Robusta Hills.
This task is quite simple, you just need to focus. You need to get into a room that is protected by an electrical safety device. It can be disabled by activating the buttons in the correct order. This combination can be learned by running around the village and finding boats that are signed with names. Right next to them are numbers. However, we will let you off the hook and give you the correct order.
The first button is next to a purple sign that says "Tigre Del Mar".
The next one is near the green signboard that says "Papi Chulo".
The third is next to a blue sign with the name "El Lucky".
The fourth number lies on the desk, right next to the yellow "Clarito" sign.
The final sign is on a metal bookcase. It's next to the name "Rojo Victoria" in red.
The correct code will grant you entry into the room. Inside you will find a chest with weapons and the mission will end once you open it.