Umibozu

One of the more mysterious yet common creatures are the Umibozu, giants that seem like black sillhouettes with piercing orange eyes, crushing the spirit of the unfortunate sailors that encounter it, or so it is said.

In all reality, Umibozu are charcoal-black skinned creatures with eyes similar to that of a Human, but with piercing, glowing orange irises. They have two distinctive lines vertically crossing each eye and going onto the neck and onto the body. An Umibozu is never higher than waist height in the water, but witnesses have reported that they do have legs, despite what rumors entail.
Umibozu typically stalk their prey (a ship) for a while from afar. It is typically spotted by the crewmembers by the glowing orange eyes in the distance in the night, but it is occasionally ignored. Once this is finished, it will now submerge and approach slowly. You cannot see it's eyes whilst it approaches, likely because the eyes are closed. When it is around 50 feet close, it emerges. Typically it hits and shakes the boat until it can effectively be dragged underwater. This is incredibly good for the Humans, as they have a chance to provide a stalwart defense. 
Typically, smaller ships have a set of javelins in the boat for this exact occasion, but their skin is incredibly tough to get through, and it typically never works unless it is a nice shot in the eye or underarm.
Other times, on larger ships than a frigate, the ships are equipped with spring-loaded Harpoons as well as javelins, but they don't always work. If you are on one of these frigates, the chance of being destroyed depends on both the experience of the crew and the amount of people attacking the Umibozu, not least the size of the Umibozu, but it ends up coming out somewhere between a 60% chance of surviving, not bad at all, and even when the ship is sunk, you can use the lifeboats in emergencies.

The Umibozu emerge at sunset, and quite before that, often, and stay lurking until well into the morning, sometimes even being spotted at noon. This means that not even the daylight is safe from these monsters of the deep. However, using a glass on the bottom of a ship revealed that they are not visible in the waters that they emerge from during the day, however, it could certainly just be deeper than what was revealed. 
Umibozu have appeared in many worlds, not just the Earthion. Some speculate that there are ways to get to the place where Umibozu and their family come from. Some have supposedly even accomplished that, like the Lady in Black and her rescuer, but there was never any confirmation, and it could very well just be myth and legend.

Basic Information

Anatomy

These creatures are completely humanoid, black skinned, orange eyes, extremely slinder Humanoids that glide along the water with grace. Their internal organs are a mess of jumbled up digestion systems and a whole lot of muscles.

Genetics and Reproduction

It is unknown if these creatures even reproduce.

Growth Rate & Stages

There are some smaller Umibozu, taking the shape of a fairly large head, barely above the water's surface, that bangs and hits the ship, but typically fails. It is unknown if these are babies, but they certainly exist, and bear no other difference to Umibozu.

Ecology and Habitats

These creatures are exclusively in the deep ocean. The closest one will see one of these is a black figure in the sunset fog off the island, or perhaps attacking a ship in the distance.

Dietary Needs and Habits

It is said that these creatures eat Humans, but this cannot be confirmed.

Additional Information

Domestication

Legends say of the Archipelogans taming these creatures and teaching them to attack and destroy Nullockian ships, but this has no basis other than oral legends and a few books full of myths.

Uses, Products & Exploitation

These creatures are quite valuable to kill. Umibozu Leather is one of the most sought after fabrics in the world. It is extremely tough, light, and fairly soft to the touch. It is so hard to cut that one man reportedly said that "to cut Umibozu, you have to have a fresh cut knife, or one of those eastern, sharper ones". 
The jelly in the eyeball is also incredibly useful not only as a material for potions but as a flammable material. To impress people, some light it and watch as it burns for days. However, this isn't perfect, as it is quite slippery, and it slides away quite easily. People put it in sacks rather than buckets, as it often slides off the decks in the buckets.

Perception and Sensory Capabilities

It is believed that these creatures have absurd eyesight, but it cannot be confirmed.

Scientific Name

Umi Umibozus

Average Height

These creatures are typically around 50' above the surface of the water but can vary wildly.

Average Weight

These creatures weigh a lot, the precise weight is unknown.