Children of the Sea volume 4 features story and art by Daisuke Igarashi.

Jim, Dehdeh and Ruka's mom finally track down Anglade's yacht, but neither the rogue scientist nor Umi or Ruka are on board. All that remains is a cryptic invitation to Jim. While Jim tries to figure out the game his ex-colleague is playing and Kanako dives into memories of her daughter Ruka, the strange seachanges continue around the globe.