The manta ray
The local divers got their gear and headed out to the pier where Peter said that a manta ray appeared. They looked around but saw nothing. They said that they were going to jump into the water and check the situation out regardless. So that is exactly what they did in the end.
The two divers jumped into the water and swam around. It was still daytime so they could see fairly well underwater. Finally, after looking around the area for a few minutes, they spotted the manta ray. It was just circling around, not doing anything else. Then, they saw the large net it was carrying around.