Perhaps no other aquatic yōkai is as mysterious as the giant umi bōzu. Their true form is unknown. Umi bōzu are only ever seen from the shoulders up, but they appear to be roughly humanoid in shape, with inky black skin and a pair of large, round eyes. Eye-witnesses report a great range in size, from slightly larger than a ship to a size so unimaginable that only the creature’s bulbous face is visible above the water. Its head is smooth and round like a venerable monk’s, and its body is nude and as black as shadow.

Umi bōzu appear on calm nights, when there is no sign of anything out of the ordinary. All of a sudden, with no warning, the waves and the weather whip up into a furious condition. Out from the tumult rises a titanic creature. It moves to destroy the ship, either by smashing the hull in a single blow, or taking it down bit by bit, depending on the size of both the ship and the umi bōzu.

Some rare reports make them out to be more serpentine, while others make them out to be more ghostly, like a gigantic kind of funayūrei. In the same way as the funayūrei, umi bōzu will demand a barrel from the crew. It uses this to pour huge amounts of water onto the deck, quickly sinking the boat and drowning the crew. If given a barrel with the bottom removed, the umi bōzu will scoop and scoop to no effect, and the sailors will be able to make a lucky escape.

Some say that the umi bōzu are the spirits of drowned priests, cast into the sea by angry villagers (this may also be implied by their name). These priests were then transformed into ghosts due to the horrible nature of their death, making them cousins of the similarly dreaded funayūrei. Others, however, say that umi bōzu are a sea monster which lives in the deeps of the Seto Inland Sea, and that they are the progenitors of a large variety of other aquatic yōkai. Because sightings are rare and almost always fatal, it is likely that the true nature and origin of this spirit will remain a mystery for a long time.