Umibōzu.

Umibōzu (海坊主, うみぼうず) is usually seen only in part. Sailors describe an enormous black form rising from the sea, with a smooth bald head like that of a monk and a body that remains mostly hidden under the water. Because so little of it is visible, the creature stays unstable in imagination. Some reports make it purely humanoid, others more ghostlike, and some early modern texts edge toward turtle-like or sea-monster analogues.

That uncertainty is central to the yōkai's power. Umibōzu is not a neatly bounded beast but the terrifying moment when the sea itself seems to lift a person-shaped will against the boat. The dark head, the sudden scale, and the inability to see the whole body make it one of the strongest maritime images in Japanese folklore.

Umibōzu attacks vessels at the worst possible moment, when the sea appears calm and ordinary. The creature may smash or swamp a ship outright, or it may speak first and demand a ladle, dipper, or barrel. Once given the tool, it begins pouring water into the vessel until the crew is overwhelmed. That motif links it closely to funayūrei, and in some regions the two are difficult to separate.

Survival depends on knowing the rule. Some traditions say smoke drives the creature off. Others insist the correct response is to hand over a bottomless ladle so the yōkai cannot use it effectively. These encounters make the umibōzu less a character with one story than a maritime test of taboo knowledge, nerve, and the thin line between practical seamanship and supernatural fear.