74 miles East of Otaheité Achievement in Call of the Sea

Story-related; cannot be missed.

Here is a walkthrough of this chapter, including all miscellaneous achievements, notes, logs, and murals.

When you get off the boat, head left and into the cave, all the way to the end. Interact with the wooden things in the water in the distance (Note 1/7).  Leave the cave then take the left path all the way to the closed wooden gate at the end. Pull the lever on the left to raise a bag, and you can pull the lever on the right. Interact with the opening on top of the podium on the right (Note 2/7). Turn around to go back the way you came but immediately make a left, following the sign that says "Norah" to a little lockbox on a table. Interact to open it with the key you have. Interact with the page inside (Note 3/7). Head back toward the boat but make a left into the next opening, with the two colorful floral strands. Next to the one on the ground in the back, interact with the slab of rock to the right of it to find the cap for the podium at the gate. Take it to the gate, place it on the right, pull the lever to open the gate, and head inside.

Shortly after going through the gate, you'll see a little campfire on the right. Head to the wooden crate and pick up the photo on it (Log 1/2). as well as the contract of passage (Log 2/2). Continue along the path (ignoring the totem and wooden bridge off to the left for now), until you get to a fork. Take a sharp right to follow that path, around and over some boards over water, to a red piece of wood, Interact with the handle on the side to get the sun symbol (Note 4/7). Continue along the path to the left of the red wooden thing until you cross more boards and come to another fork. Make a right and follow the path to a large open area with tons of red wooden things again. On the far right, you can pull a lever to open a gate back to the starting area, if you want. Turn around and look for the red wooden thing with birds all over it. Head to it and interact with it to get the bird symbol (Note 5/7).  Directly across the area from where you entered it is a green rock ramp you can walk up. Walk to the end and look down in the water to your right. Interact with the barrel to unlock A brigantine, a brig and a barque (20G). 

Leave back the way you came in, following the path until you see an obvious partially submerged wooden house in the water on your right. Get close to the red wooden thing in the water near it and interact with it to get the fish symbol (Note 6/7). Turn left and continue on the path until you get to another obvious red wooden thing sticking out of the bush on your right. Interact with it to get the mountain symbol (Note 7/7). Turn left and continue on the path, back to the puzzle totem at the wooden bridge. Interact with the puzzle totem and input, from top to bottom, sun, birds, mountain, water, fish. A screenshot of the solution can be seen below.



Press the red button to open the bridge, but before crossing it, backtrack to the starting area where your boat landed. It's gone, so interact with the marks in the sand to unlock Gently down the stream... (25G). Now you can head back and cross the bridge to end the chapter and unlock this achievement.