One of Deep Sea Demons types easily identified by their fish-like appearance and their occasional attack of fisherman. They are often called by crows of the sea, due to emitting a noise similar to these birds.
All of them seem to be alike marine animals like fishes, crabs and squids.The first sea ghost shown looked somewhat like a fish wit a crab's short feet and a rounded mouth as large as its body and filled with dense teeth. It also had a pair of furry arms divided into five fingers, like human hands, on both sides of its mouth.  Later, a bunch of fish-bodied crab-legged creatures which resembled giant cockroaches with human-like arms attacked the ship Charming Beauty along with a giant tooth shark octopus amalgam. When in a group they appeared like a nest of tadpoles in a puddle, but in a much larger scale, capable of dyeing the entire sea black.
Initially seen by the Fjords as monsters disguised as fish who bite and drag fisherman into the water, Thunder believes them to be simply Demonic Beasts. After the underground exploration of the Great Snow Mountain, they were eventually associated to a new sentient civilization, the Deep Sea Demons. After learning of the origin of the Demonic Beasts and Demonic Hybrids, they can either be considered as a water based variant of them or as another species altogether. Nevertheless, they are known to the demons as the lowest constructs of the Sky-sea Realm, with their only advantage being their numbers. Due to this, they are most suitable for filling the battlefield.
The term was first mentioned by a sailor after seeing a fish-like monster, while Tilly Wimbledon explored the previously Shadow Isles of the Fjords along with Thunder.  They are then seen by Tilly when she travelled to Border Town via Shallow Beach for the first time, eventually killing a bunch of them with the help of Shavi, Andrea and Ashes.  Though they were known for a long time by humans who traveled the Ocean like the Fjords people, it only started to be associated to a sentient civilization after the underground mountain exploration. 
Joan was initially believed to be a Sea Ghost by sailors due to her scales covering her cheeks, neck, arms and legs.  Thunder recounts to Roland the death of a friend who got killed during a Sea Ghost attack, during Lightning's awakening.  During Thunder's exploration of the Ocean further away from the continent than the Sealine, they were attacked by Sea Ghosts  and other Deep Sea Demons after apparently arriving close to the Sky-Sea Realm territory. After sometime fleeing, Devour appears to have intervened and attacked them to evolve himself. After being stranded from Thunder's exploration while still in the Fjords, Joan encountered several Sea Ghosts and other Deep Sea Demons and though attacked by them, managed to escape.
When entering the Bottomless Land after the Demon Civilization and Human Civilization's truce, countless sea ghosts propped up their heads over the sea as they cawed at the floating island. It was as though dense black pores had bloomed on the sea surface. Just sweeping across it with one's eyes left one's scalp tingling. The bodies which were floating beneath the water made the seawater appear a strange black color.
After the Custodian agreed to stop the Battle of Divine Will and step down from his position, a great magic ripple escaped from the Bottomless Land and made every Blade Beast and Nest Mother collapse to the ground, leaving only the Sea Ghosts unscathed. After it, free from the Custodian's control they receded like the tide.
- No ships seem to be able to outstrip a sea ghost.
- The creation of this Deep Sea Demons' type might have some real world inspiration since it harks back to sea monsters often mentioned in literature like Kraken, Hydra and Loch Ness Monster.
- Along with the Demonic Beasts and some of the Demonic Hybrids, the Sea Ghosts are one of the few species of the Deep Sea Demons still alive after the end of Battle of Divine Will.
- Chapter 232
- Chapter 324
- Chapter 1481
- Chapter 816
- Chapter 1418
- Chapter 954
- Chapter 1016
- Chapter 1216
- Chapter 1264
- Chapter 1294
- Chapter 1492
- Chapter 1216