Seafolk

From the perspective of land-bound scholarship, the people of the sea are a mess of mixed bloodlines. The seafolk, on the other hand, put little stock in the clarity of lineage.
Those who are called seafolk are the children of a dozen waterborn lineages, and self-identify based less on birthright than on phenotype. No record exists of who the original oceanic lineages were, with the exception of a faction of the Sea Elves who favored life beneath the waves. There is a considerable degree of debate over whether the sea elves count as seafolk. They are definitely kin to the modern seafolk, but have kept much of their original culture intact and separate from the blended lines of the seafolk 'proper.'
The seafolk have a broadly homogenous culture, with variations by geographical region. They distinguish four lineages by rough morphology and by the depth at which they dwell.

The Tritons, also known as the shorefolk, are the most amphibious of the seafolk, living equally above and beneath the waves. They resemble Elves, but with skin in deeper, maritime hues, webbed fingers and toes, and fin-like ridges around their joints. They are the liaisons between the sea and the surface, custodians of the upper cities - reef-and island-built cities which form the capitals of the great oceanic kingdoms - and as such, often come to think of themselves as the aristocracy of the oceans.
The shoalfolk differ from the tritons in many ways. Also called Merfolk, they have many traits in common with surface lineages such as the elves or Humans, but have powerful, fluked tails and extensive fins. They can breathe air or water, but cannot walk on land, save with the use of Magic or prosthetics. They are, however, powerful swimmers, especially the small proportion who possess a double tail. While the tritons are focused on governance and trade, the shoalfolk provide the bulk of the seafolks' food, in the form of fish, cultivated kelp and shellfish of various kinds.
The deepfolk have more in common with sea creatures than with surface lineages. They are warm-blooded, with skin like a dolphin's, but also fish-like fins and scales. They have flat, scaled faces, black eyes and teeth like a handsaw, tails with an upright fin, and keen senses. Powerful swimmers, they guard the trenches which might lead to The Abyss, and consider themselves kin to sharks. The deepfolk and the shoalfolk are neighbours, rivals, and allies, according to need and temper, and the same is true of the deepfolk and the divefolk, who dwell in the deepest ocean trenches, mining their riches and avoiding the evils that the deepfolk drive back from the surface. The divefolk are a fishlike, cold-blooded time, with pale scales and huge, dark eyes, although traces of surface blood may linger in individuals. They are an iso­lated and insular folk, and in addition to a traditional rivalry with the deepfolk, they despise the 'shallowers' who demand the riches of the deep without venturing into the trenches.
The Koalinth are not truly part of the sea folk, but share their environment. They are goblonoids - primarily of hobgoblin stock - who have only recently adapted to aquatic living by developing webs and gills.
The trenches are also home to the threat that the divefolk avoid and the deepfolk guard against. The Merrow were once seafolk who fell victim to the corrupting influence of Colossal relics which were lost in the deeps or bubbled up from the Abyss. Some stories say that they allowed themselves to be changed in order to become stronger warriors against the same horrors that eventually took them.
The merrows are now huge creatures, twice the size of the deepfolk. Not unsophisticated, they are however held by a blood-deep rage, which wells up in battle as a terrifying fury. They shun the light, but on moonless nights may venture up to attack the upper cities. Al­though they can be destructive, these raids do have the effect of suspending the otherwise constant infighting between the oceanic kingdoms, and between the four seafolk lineages.