A 'Funayurei' is a Japanese ghost, the vengeful soul of someone lost at sea. They are characterized often as an undead water-logged corpse, a gigantic black humanoid (Umibozu), and sometimes a false ship or fire meant to draw unwary vessels further from shore.
My concept for this is a gigantic corpse, one made by multiple smaller ghosts merging together into a larger entity. Due to the depth of the ocean, it can grow to enormous heights without being noticed, and walk along the sea floor in an endless search for something they have lost.
They are incredibly aggressive, and seek to destroy ships in some way that ties in with how they have died. In order to see above water, they use destroyed ships as masks to hide their heads. If one Funayurei encounters another, they will slug it out until there is a winner, which absorbs the broken apart ghosts to become more whole.
Some Funayurei are a mangled mess of different people. Some of them resemble one very strong soul that guides the mass.
All are driven by vengeance, loneliness, or some deep pain that they can never fill.