Umibozu

The Umibozu is a Japanese Yōkai whose true form is unknown. They appear human in shape with inky black skin, large round eyes and bald heads. They are only seen from the shoulders up probably because they are just that big. They are easily the size of a ship if not much larger. They appear on calm nights and cause bad weather and rocky seas to pop up suddenly and without warning. Some say they are somewhat ghostly. They are sometimes believed to be the souls of drowned priests that were thrown overboard. It was thought they would demand a barrel so they could scoop water into a ship to sink it. This can be counteracted by giving them a barrel with a hole in the bottom. They have been kept mysterious by the fact most encounters with them end in death.

Citations:

Meyer, Matthew. The Night Parade of One Hundred Demons: A Field Guide to Japanese Yōkai. United States, Matthew Meyer, 2015.