In your notebook, after unlocking the "Voice of the Night sky" quest, this sub-quest will appear in your notebook.
After you've collected the twelve zodiac coins mentioned, A strange voice will come over the radio and your player character will write down in the notebook:
"The mysterious voice said "At the edge of the end of the crimson waters"...Check it out with Oceana."
What happens next is that you and Oceana go to the Zahhab Region. At the bottom right corner of the map, when you swim to the limits of your diving range (horizontally), a cutscene will commence, showing the legendary, thought to be long-extinct Cameroceras swimming past your diving range. Oceana will ask if she's gone mad (not illogical, considering the presumed extinction of the massive Cameroceras).
When you go back to the island, your diver will have written in the notebook:
"What was that... Were you seeing things?"