The Criptograma Chests are a new feature in the Far Cry series that requires you to find two Criptograma Charts to unlock a chest with some good loot inside. One of these chests can be found in the town of Poesia in the Costa Del Mar region. Use the map below for reference to where the area is. It is located on the western edge of Yara. It is easily reached by fast traveling to the Colinas Plains Checkpoint if you have it unlocked.
When you reach the town, search around for a dock that wraps around to the back of a house. There may be two NPCs fishing at the end of the dock. The chest is there with an arrow at the end of the dock pointing out to the ocean.
Jump into the water and swim your way over to the small rocks you can see in the distance that is to the right of the island. There may also be a jetski that you can use to reach the rocks. Go around to the other side of the rocks to find the first Criptograma Chart.
There is an arrow behind where the chart is located that points to the next chart. Swim or use a water vehicle to go over to the island with the lighthouse on it. Stick to the right side of the island and travel through the water until you come across a beach. To the right of the beach, you will see another arrow pointing up.
Look up to find a grappling point. Use your grappling hook to climb up, and the Criptograma Chart will be at the top.
With both of the Criptograma Chart collected, head back to the chest to obtain your reward.