The Islanders know one thing: they have no purpose. They survive merely because of a human, instinctual fear of death. They survive because they love each other. Some of them survive because they believe they had a purpose in another life. Some will eventually give up on survival itself. Above all, they survive because of Aqua's dreams, which tell them that the outside world is not lost--yet.
Sereia, Isla, and Naia enjoyed their ordinary life in Australia. But when they get stranded on an island and stumble upon a sea cave, everything changes. Land or sea, they grapple with their newfound abilities while helping out friends and defeating foes.
(also posted on AO3)