This is in Chapter 5. Once you lower the water for the first time you will enter a portal underwater. You should see a tomb like structure near the top of the water. It will have a prompt on it to open a timed door. Facing that look left and swim straight down to see a small rock table next to some red coral plants. (To the right of a hut as well). Go to the table to find a Tiara. Interact with it to receive the achievement.