A person... encountered in the sea at night?

Below is the world setting.

The stage is a certain isolated island. On land, there are humans. And In the sea, there are merfolk, jellyfish that can take human form, and mysterious fish. Of course, there are also those who prey upon them or are preyed upon.

Basically, there is little conflict.

The upper area of the sea is a coral reef. There are merfolk, fish, and various colorful, mysterious corals.

The middle area receives little sunlight, and it is said that jellyfish mainly inhabit this zone.

The lower area is not completely dark, but very little sunlight reaches it. It is said that giant fish that prey on the inhabitants of the upper area live here.

Occasionally, items from the human world fall into the sea, and there are many who collect them.