Ikuchi (イクチ) is a sea monster most often portrayed as a massive serpent that are covered in slick oil. These monsters can stretch over many kilometers and inhabit the open seas near the coasts of Japan.
Many stories exist about the Ikuchi, primarily from the Edo period, but they all have a common theme. A ship is sailing peacefully when they suddenly meet an Ikuchi. The Ikuchi wraps itself around the ship and slowly slithers it’s massive body along the deck. Sometimes this could take hours or even days because of how incredibly long the Ikuchi is. During this time, sailors would have to near constantly bail the slimy oil from their ship or risk capsizing due to the added weight of the oil.
This legend sort of reminds me of the Loch Ness monster, though perhaps a bit less benign. However, I definitely would not want to meet either one if I was sailing the open seas; I wouldn’t fancy encountering any sort of giant sea serpent!