From the Deeps and its islands, the Deepworlders emerged. While surface and island deepworlders usually look fairly humanoid with turquise, scaly skin and visible gills and fins as extensions of their limbs, Deepworlders from further below tend to look more distorted, with tentacles instead of hair (jellyfish- or even octopus-like), somewhat watery/'transparent' skin, sharp teeth, or other uncanny things waiting in the Deep. They can reach from magnificence to total horror.  

Residents of land and seas

There are different types of Deepworlders, all originating from down in the Deep, but having changed after building their homes further above.
Islanders are the most human-looking Deepworlders, with only some gills and fins as extensions of their limbs, but nothing too distorted, as they mostly live on the lands other than in the seas.
Midlanders, on the other hand, live in the Deeps, however not very deep down. Their skin is scaly, like that of fish, and their gills and fins are way more visible. Often, their hair resembles jellyfish-tentacles.
And then, there are Deeplanders. Creatures, most people will never get to see in their life, as only a few of them ever see the sun. They often have octopus-like tentacles instead of hair on their head, bright, way-too-colorful eyes, and sharp teeth.  

Mark of the Deep

Especially Deeplanders are prone to something called the "Mark of the Deep", a disease that spreads among deep sea creatures, giving them distorted features like too-long fingers, an extra eye or limb, a split tongue, teeth on one's skin, or other uncanny things one can imagine.
There may be something lurking down there in the Deeps...something evil.