The first thing to jump to mind were water-nymphs. The problem with these is that playing a nymphomaniac fey critter tied to a single spring isn't much fun. Also, male PCs are left out in the cold.
I then thought about merfolk or tritons. The two problems here are that merfolk and tritons usually have less than desirable men-folk, and they are definitively tied to the ocean (that is, they always have fish-fins instead of legs).
The next major possibility is selkies or merrow. For those unfamiliar with these more obscure mythical critters, they are shapeshifting waterfolk who sometimes can be found on land. The selkies have a seal skin that they can put on to go swimming, and the merrow have a magic cap or cape. This has a lot of logistical appeal to it for fitting into a mainstream adventuring party. The one issue I have is with their personality, which really tends strongly towards the fey (too sanguine).
So, here's my reskin of the Merrow. It is heavily inspired by the mythos of Atlantis, and one could weave Atlantis into one's campaign world for a lot of pseudo-Western European or classical flavor if desired.
Some say that they [the inhabited regions] begin at the beginning of the western ocean [the Atlantic] and beyond. For in the first days there was an island in the middle of the ocean. There were scholars there, who isolated themselves in [the pursuit of] philosophy. In their day, that was the [beginning for measuring] the longitude[s] of the inhabited world. Today, it has become [covered by the] sea, and it is ten degrees into the sea; and they reckon the beginning of longitude from the beginning of the western sea. - Ancient Hebrew Text
Patient & reclusive, depths of wisdom; heirs to a City beneath the Sea.
Average Height: 5'4-6'1
Average Weight: 120-140 lb.
Ability Scores: Favor Wisdom or Charisma
Speed: 6 hexes
Languages: Lingua Acquea
Merrow are an ancient race known for their kindness, depth of spirit, and curiosity. The Merrow are said to be desecended for the people of an ancient city that sunk beneath the waves in the first days of the world. They are a shy, patient people who have seen the coming and going of many glories, and it is difficult to rouse their ire; but they make reliable and compassionate friends and capable diplomats or administrators.
Play a merrow if you want . . .
✦ to be the wise heir of an empire long lost beneath the waves.
✦ to be at home in the seas.
✦ to be a member of a race that favors the paladin, monk, priest, beguiler, or skald classes.
In the height of all these doings, what should there be dancing among the outlandish set of fishes but a beautiful young woman - as beautiful as the dawn of day. She had a cocked hat upon her head; from under it her long green hair - just the colour of the sea - fell down behind, without hinderance to her dancing. Her teeth were like rows of pearl; her lips for all the world looked like red coral; and she had an elegant gown, as white as the foam of the wave, with little rows of purple and red sea weeds settled out upon it: for you never yet saw a lady, under the water or over the water, who had not a good notion of dressing herself out. - The Wonderful Tune, Irish Tale
But what was his astonishment at beholding, just at the foot of that rock, a beautiful young creature combing her hair, which was of a sea-green colour; and now the salt water shining on it, appeared, in the morning light, like melted butter upon cabbage. - The Lady of Gollerus, Irish TaleMerrow are a people much like comely humans that live below the waves. Physically, Merrow look much like normal humans; however, they usually have slight webbing between their fingers, and often have long green hair.
As a child, each Merrow is given a cohuleen driuth, or magic cap, which allows them to swim freely beneath the ocean as a fish. The process for making these is a closely guarded secret; some say it is an arcane secret from the first days before their city sunk into the oceans, others say the techniques for making them are handed down from the Sea-Gods. If a Merrow loses their cap, they must remain in the world of men until it can be found again.
Merrow women are about the same size as the men-folk, and women play a central role in Merrow society. It is not uncommon for a woman to be the high chief or priestess of a band of Merrow. However, these undersea people also embrace duality, so it would not be unusual for the leadership of a group to be shared between the sexes, even if the male's duties are largely symbolic.
Playing a Merrow
But at a later time there occurred portentous earthquakes and floods, and one grievous day and night befell them, when the whole body of your warriors was swallowed up by the earth, and the island of Atlantis in like manner was swallowed up by the sea and vanished; wherefore also the ocean at that spot has now become impassable and unsearchable, being blocked up by the shoal mud which the island created as it settled down. -- Critias
Merrow are heirs to an ancient legacy that has seen greatness and fallen from glory. They know that there is more to life than the surface-worlders crave, and they lead spiritual and patient existences. They are comfortable with complexity and duality and seek to understand the world through calm reflection as well as curious and open-minded questioning. As heirs to a great metropolis, Merrow maintain many traditions that allow them to be comfortable administrators and diplomats.
Outsiders criticize Merrow for being lazy, resistant to change, or unemotional. However, Merrow know that the tide will eventually wear away any wall built by man; all things, in time, will pass. Thinking the Merrow can be taken advantage of easily, however, would be a mistake. Merrow are compassionate and reliable friends. Their culture is ancient and their elders are wells of deep wisdom. While they are slow to anger, once the Merrow have set their mind to something, it will likely come to pass.
Merrow Characteristics: Calm, unemotional, self-content, kind, shy, consistent, relaxed, rational, curious, observant, reliable, and compassionate.
Male Names: Evenor, Atlas, Eumelus, Gadeirus, Ampheres, Evaemon, Mneseus, Autochthon, Elasippus, Mestor, Azaes, Diaprepes
Female Names: Leucippe, Cleito
Leucippe is a fair maiden from beneath the waves who, as a priestess, is the spiritual bedrock for her band of adventurers. Motivated by ancient tales of her people's lost land, she is particularly interested in uncovering any clues to the location of the city below the sea. Her curiosity for lost artifacts or forgotten tomes is insastiable.
Atlas is a strapping knight from beneath the sea. His family is said to be descended from the blood of kings and his noble bearing is testament to this lineage. While not the boldest or most innovative warrior, Atlas is devoted to his friends and is always there for them when they need him most. He is slow to anger and has a great reserve of patience. Many years ago he lost his magical cap; deep inside, he secretly yearns to return to his home beneath the waves. For now, he settles for being the captain of a small sailing ship.
Mestor is an ascetic monk from the Northern oceans. In his youth, he traveled widely as a sailor and dallied with women in every port; however, as he grew older, he became disillusioned with his life. Mestor became an ascetic, giving away his possessions and hurling his cohuleen driuth into the sea. He still travels with ships as a navigator, and his depth of experience, know-how, and network of contacts is valued by many captains. Mestor knows that when the time is right and he has learned what he is to learn from the surface-world, fate will bring his cap back to him so that he can return to his people.