A sea monster which appears frequently in coastal folklore and Edo period writings. It is notorious for wrecking boats and dragging people into the sea or, like its kin the funa-yūrei, asking with dire intentions for a ladle for bailing out boats. While it takes various shapes, is most commonly conceived as something huge and pitch black with ambiguously human features and a common lack of eyes or hair.

A well-known umibōzu tale appears in the Usō Kanwa 雨窓閑話, a collection of stories from the late Edo period:

A noted sailor by the name of Kuwana no Tokuzō 桑名の徳蔵 violates the taboo on boating at the end of the month and goes out to sea alone. Sure enough, he encounters a great nyūdō (a euphemism for a bald-headed monster) 1 jō (~3 meters or 10 feet) in height, with terrible eyes like scarlet mirrors. The ōnyūdō asks Tokuzō if he finds its shape frightening, but the sailor replies that he finds nothing frightening but making his way in the world. With no reply to this wit, the umibōzu vanishes instantly. ¹²


Citations
1. Murakami 2005 pp. 50-1.
2. Sasama pp. 188-9.