Drifting Home

Raised like brother and sister, Kosuke and Natsume have been friends since childhood, but their relationship begins to strain in sixth grade after Kosuke's grandfather Yasutsugu passes away. One day during their summer vacation, Kosuke and his classmates sneak into an apartment complex that is scheduled to be demolished, and rumored to be haunted. Both Kosuke and Natsume grew up there, so the place holds a lot of memories for them. There, Kosuke stumbles into Natsume and is asked if he knows about the mysterious Noppo. But suddenly, they get caught up in a mysterious phenomenon. When they regain consciousness, they see a vast ocean before them. As the apartment complex drifts in a mysterious sea with Kosuke and the others on board, they band together to try and survive. There are tears and fights, and maybe even reconciliation.

Synopsis from Anime News Network