The hint at the telephone booth is telling me that I need to dive into the mouth of the catfish temple in the middle of Martha’s Bay. I have found the temple entrance that I think this is referencing but It is surrounded by stones and I can’t get into it. Diving only lets you go down below the surface a small bit. It isn’t enough to swim down to get under the rocks. How do you get into the catfish mouth?
Look to the left side of the catfish building in the water. There will be a square of water that looks a bit different to the others in the area. Dive down on this tile of water and you will enter a new area where you can properly dive to the entrance of the temple.