The Ocean Below

The Ocean Below lies at the 'surface' level of the Sunken Islands, and has many notable differences from the Ocean Above.
Firstly, the tides are dictated by an unknown force; with the absence of a sun or moon, it's unknown how much impact the natural cycle Above has upon the Ocean Below. Few routes have been established -- and when they are they rarely stay the same, as the tidal directions often shift and change; fisherman keep to the local shores known as the Shallows, and even then it is easy to be swept up in riptides or rogue waves.
As of present, only one ship has been built that's said to be able to traverse across the Ocean Below, but The Godtooth has yet to set sail and prove this claim - for now, the only things that traverse the Ocean Below freely are the predators hunting in the dark. Whaler ships still hug tightly to the shores, never daring to leave the Shallows. 
The water of Below is more akin to ink or tar in colour and texture than typical oceanic water; slightly thicker , it's known to sailors that jumping in can often lead to being dragged down to the bottom by the sheer weight of the 'ink-waters' clinging to clothes and skin. The inability to see under the surface, and the dangers of ever getting under it, have rendered the Ocean Below a dangerous and inhospitable place to all except the most adaptable of creatures.

Geography

The majority of the Ocean Below is unmapped; it's known that several islands exist within it, though their precise locations and distances from one another is often disputed. The only mapped area that cannot be argued about is the Shallows, though even this ring around The Carcernatorum is treacherously deep in places.   While still being deep enough to comfortably sail warships through without any chance of scraping the hulls, the Shallows are called thusly due to the fact that it's considered a nursery for some of the younger predators inhabiting Below. The lack of fully grown predators makes for lucrative fishing, though it still comes with it's risks - as all things Below do.

Ecosystem

Due to the high pressure from the depth of the Sunken Islands alone, let alone that of the deeper parts of the Ocean Below, the majority of creatures that live within it have evolved to live and survive within the inky waters. Deep sea gigantism has resulted in colossal whales, crabs and a broad variety of other creatures that have adapted to fight and kill in the harsh biome. 
While most of these creatures are impossible to hunt with even a fleet of fishermen, washed up carcasses from territorial fights or feedings have provided the people of The Carcernatorum with resources and materials; it's said that the Southmarchers built their original home with the beached carcasses of Broadsnout Whales.
The plant life can be equally as dangerous to man and creature alike; reefs are alive with unknown dangers, though it's rare that anyone manages to catch a glimpse of the world that exists underneath the surface.

Ecosystem Cycles

During the Passage of the Lilies, the release of pheramones in flowers that float in great bunches across the surface tend to herald a higher aggression in sea creatures; it's thought that this is a mating season of sorts though it's not clear why it affects all marine life within the Ocean Below regardless of species, as there's little research on the life cycles of the creatures that are known to reproduce. However, the spike in aggression is also accompanied by a spike in size; brave fishermen reap a larger harvest, if they're willing to risk their lives to bring it ashore.

Localized Phenomena

The Breaches exist most visibly on the Ocean Below, wherein the connection to Above is more visible as a great towering tornado of water that climbs upwards to connect with the Ocean Above. These Breaches are too far from most Islands to be of any immediate access, however they're considered the most powerful natural phenomena both Above and Below.

Type

Ocean