Funayūrei (“marine spirit”) are ghosts of people who have died at sea. They are sometimes depicted as scaly fish-like humanoids and some may even have a form similar to that of a mermaid or merman. They approach people on boats and ask to borrow a Hishaku (a utensil for scooping up water). If they are given a ladle, they will pour sea water into the boat until it sinks.
Illustration © 船幽霊 | コウノ [pixiv]