The Cetus Coin, a single Constellation Coin out of the one hundred that can be collected in Endless Ocean: Blue World, is one of the six needed to complete the Voice of the Night Sky quest "The Hero Perseus". It can be found in the Gatama Atoll area, near Blue Cliff at coordinates A-7. In this shine spot, there are several small fish making their homes: a pair of Singular Bannerfish and a single Bluelashed Butterflyfish.
- The constellation upon this coin depicts the mythical Cetus, which is either a giant fish, whale, shark, or just a sea monster with no determining features that defines it as anything but a monster.
- The word "cetacean", meaning "relating to whales", comes from the name of this mythical monster. This means that the location of the coin could be in reference to the fact that Humpback Whales can be seen in a cutscene near the Blue Cliff area. As well as that, the Spring Garden area is nearby, which is also home to Humpbacks.
- Cetus was the sea monster that was set to devour the princess Andromeda as punishment for her mother Cassiopeia's reckless boasting about her and her daughter's beauty. The monster was slain by the hero Perseus in the end, saving the princess.