Masakazu Ishiguro 
Heavenly Delusion, Volume 2 [EPUB ebook] 

After heading south to investigate the place known as Tomato Heaven, Maru and Kiruko head back to Tokyo via a “ferry”. But they are ambushed at sea by one of the man-eating Hiruko monsters. This specific beast is amphibious, making him extremely deadly as he can easily take cover in the sea. Meanwhile in the school, Tokio, with the help of his classmate Kuku, uncovers a frightening secret. Maybe life is not what it seems at all. Maybe those who are inside the walls are not in heaven. And maybe the walls are there to keep them all inside!