After you solve the mystery of Captain Bell's ship, you pass through the strait at Bellcola and head north.

Navigate the Islands

Go east, then slightly south to where there is a small island with a single palm tree. Go to the south edge of that island, then go east to find a hidden tunnel through the big island. On the other side, go east. You can optionally use the dock, then go into the hut to find the village of Howduyadu-cola. When you're done, exit the village and get back into Sub-C. There will be a cutscene.

Follow the Signal

Nav-Com tells you that the signal is detected. Go northwest first. You will eventually find a patch of dark water. Go onto it and press B to dive. Then go to the easternmost dark patch and dive there. Go southwest and dock at the island, then go down the stairs. Take the big heart. Then go upstairs and get back in Sub-C.

Go clockwise to the east side of the island and then go east, then south to find an island with a single palm tree on it. Go east from that island, then south so you are up against the pale water, and push east through the eastern island.

Go south and you find some islands in the shape of a cross. Get on the dark water that is to the right of the leftmost patch of dark water and press B to dive. The correct patch of dark water is marked with a 💥 in the diagram below:

               🏝️
🌊🏝️💥🏝️🌊🏝️🌊🏝️🌊🏝️
               🏝️

Go south and go onto the southern patch of dark water. Dive there, then go west and dive into the dark water there.

Go north and Nav-Com will begin to give you updates. Go north along the west side of the island. Go up against the west side of the island and push east to go through a hidden passage. Then go northeast, and use Nav-Com's message to find the location of the signal. When you're on the correct spot, Nav-Com says that the signal has stopped. When that happens, press B to dive.

Explore the Underwater

Go north and you will reach a dark tunnel. Keep going north through darker and darker rooms. Eventually you emerge in some underwater ruins. Go east, then north through the door, to enter the first cave of Chapter 6.