Some info on how the world works. 

This is somewhat inspired by the game dredge. 

There are multiple different categories for what species you can be, human, marine folk, aberration, or monster. 

You have sub-categories as well, like for the marine folks, the sub-categories are sirens, merfolk, or hybrids.
Merfolk are the most common, these are your typical mermaids and merman, they have a colourful fish tail with the upper half being a human body with fins replacing their ears, their diets are similars to humans but they also eat raw fish, coral and underwater plants. 

Sirens look similar to the merfolk, except they have sharp teeth and they eat humans, their diets are the same as the merfolk but have a preference to meat. Sirens use their voice to lure humans and drown them. 

Hybrids are a mix between humans and fish, the have a human body but with an added tail and fins, the colour and shape of their tails depends on what kind of hybrid they are. Depending on what hybrid they are, they might have extra perks as well, for example, angler fish hybrids get a light bulb antenna to see in the dark. They are omnivores and eat anything edible that they can find. 

Aberrations are like the marine folks but different, more creepy looking and just a bit odd. For example they would have 5 eyes too many on their gooey mushy flesh, or they would have a mouth where their hand should be, or they have another mouth inside their mouth, or they would have their skin melting off their skeleton, or they would be glowing neon under their scales, etc. 

The severity of oddity can vary from as weird as having leaves and flowers growing out your skin, or something simple like having seven fingers instead of the usual five. But they all have one thing in common, their blood is more viscous and darker than any other creature.

Their diets are affected by how the aberration works, for example someone who has a vortex as middle, need to have their food cut or squashed to eat. Or someone who has acid in its stomach, can eat metal and rocks and have it dissolve by the acid.
Aberrations can be any of the sub-categories from the marine folk, but most of them are aberrated hybrids. 

Monsters are, well, monsters. They are like the squid thing Gem has built on hermitcraft. Massive scary creatures that rely on instinct. They live in underwater ravines and caves, little is known about these creatures other than they are carnivores and eat whatever disturbs their territory. It's why the marine folk don't live near ravines and why pirates avoid sailing above deep water, because monsters are terrifying.
It's a popular superstition that some monsters have a connection with the ocean gods, so it's not unusual for sailors to drop sacrifices in the waters in hopes for better weather. Sacrifices can be materials like gold or maps, or living beings that they bring along or find. 

Marine folk and aberrations are humanoid creatures and think like humans as well, they are intelligent and have their own civilisations. Well, marine folk do, the aberrations live on their own. 

Marine folk and aberrations can both transform into a more humanoid look if they want, though aberrations can't hide some features, some even get side effects for transforming, though the effects don't tend to last long. 

Merfolk like to live in the coastal area, close to civilisation. They like socialising with the humans that live near. Though they don't like living close to sirens. 

Sirens inhabit the water near unclaimed islands, waiting for some unfortunate pirates to come along and be their prey. It's a rare sight to see sirens living with other creatures as they don't like the company. They live by themselves but sometimes they allow other sirens if they like each other enough. 

Some hybrids prefer the solo life, but most enjoy living amongst humans and other merfolks. It depends on what type they are, small fish like salmon and cod tend to stick in schools while sharks and eels prefer being on their own. Though some small hybrids can be found alone on land as there's less predators for them there. 

Aberrations are outcasts, all wildlife like to stay far away from them since they are just freaky looking. So they can be found really anywhere, most hide in dark places like ravines and caves since the monsters don't eat aberrations. 
Even their own kin will find some excuse to not interact with each other. Example,
"He's barely aberrated, all he has are some extra fingers! He got lucky in life."
Or,
"Man she's such a freak with her glowing skin, I wouldn't want to talk to her even though my skin is melting off my bones."
Or they're just shy and don't know how to start conversations so they avoid others, whatever their reasons are, they are loners. 

Monsters live in deep water with the aberrations because they can't fit anywhere else, they rest in ravines as they attack anything that moves unless it's aberrated, for some reason they are fine with the aberrations. 

Even though sirens and merfolk don't get along, it's possible to find a siren hanging around with merfolks, though it's not very probable. 

Now some doodles I made. Ignore how crappy they are. 

Some aberrations, only two because I got lazy. Also crappy.