The Atha'an Miere, called the Sea Folk by commoners, are a group of people with complex customs that live on ships and on islands in the Sea of Storms. They only showed up on the mainland to trade goods, until prophecy seemed to be fulfilled (see Coramoor, below). They typically have a dark complexion, and are seen as "exotic" by mainlanders. The rank of a Sea Folk can be determined by both the number of piercings (typically ear and nose, with a chain in between for higher ranks) and the quality of the metal used for the earrings/nose rings.
Sea Folk porcelain sculptures, delicate and graceful objects of art, are highly prized by mainlanders, frequently costing their weight in gold to obtain.
Creation
During the Breaking of the World, everyone was a refugee, fleeing from the destruction wrought by the Male Aes Sedai and society collapsing. While some fled inland, moving as the landscape changed and water moved, others fled to the ships. Despite not knowing how to use them or knowing anything of the sea they soon mastered their boats, and also made their promise to the Amayar.
Windfinders
Windfinders can be found on every ship the Sea Folk inhabit. Most Windfinders can channel, and are ranked only under the Sailmistress and Cargomaster on a ship. Windfinders who can channel can typically handle Air and Water most skillfully, creating thick and powerful flows to aid their ship's travel. Conversely, many have little to no skill with Fire or Earth, even less than most female channelers, due to the nature of their society.
Windfinders typically fear being found by Aes Sedai, as they believe the Aes Sedai may take them to the White Tower against their will to train them. Because of this, a Windfinder has the right to refuse passage to a passenger who can channel, something usually reserved for the Sailmistress of the ship.
However, when Nynaeve and Elayne approach the Sea Folk on the orders of the rebel Amyrlin Egwene al'Vere, they comply with many of the Aes Sedai's requests, such as assistance in finding and using the Bowl of The Winds and helping Elayne Trakand maintain Caemlyn's food supplies during the siege, by opening gateways for merchant wagons to travel through.
Culture
The Atha'an Miere are headed by a Mistress of the Ships, roughly equivalent to a queen, but without a fixed geographic territory. Her consort and/or male counterpart is the Master of the Blades, responsible for trade and security concerns. Under this leadership, the Sea Folk are further organized into clans, headed by a Wavemistress. It is unknown at this time whether there is a clan-level counterpart to the Cargomaster.
A much lesser-known part of Sea Folk culture is that their vaunted porcelain is actually created by the Amayar, land-dwellers on the island of Tremalking, overseen by a Sea Folk governor. The Amayar apparently follow a pacifistic lifestyle very similar to the Tuatha'an Way of the Leaf.
Another little known facet of Sea Folk culture is that both genders go topless while at sea, and out of sight of the mainland. Presumably all of their passengers know this eventually, and keep silent either out of respect/moral outrage, or for more prurient interests. This fact alone may account for why passage on a Sea Folk ship is frequently so costly, although they are also the fastest sailing vessels in the land, and some of the most reliable (partially due to the unrevealed abilities of Windfinders).
The Amayar
Unknown to any but the Atha'an Miere, there are a group of island dwelling people that they protect. During the breaking they asked the Sea Folk to protect them from the madness of the Breaking, as men stole and slaughtered, and they take this as a solemn duty. It is the Amayar that make the fabled Sea Folk porcelain, and also their trade goods. In counterpoint to the Atha'an Miere, they rarely leave land, and, if they do, it is only in small fishing boats that stick close to shore.
In Knife of Dreams devastating news is delivered to the Mistress of Ships during her meeting with Logain. The Amayar have committed mass suicide in accordance with their prophecies after the giant crystal hand on one of their Islands has melted. Children were given instant poison, and the adults used slower poison. There are almost none left alive, and the few that have been found are said to be immensely sad at being denied death.
The Coramoor
The Sea Folk believe that the Coramoor is predicted by the Jendai Prophecy to lead them to greater glory, including domination of all the seas of the land. Rand Al'Thor, who is also the Dragon Reborn and the Car'a'Carn of the Aiel, is believed to be the fulfillment of that prophecy. The Sea Folk will serve the Coramoor in any way they can, but no-one agrees on exactly how the Bargain should pan out. By Knife of Dreams an agreement has been reached, with particularly onerous terms both ways, something which the new Mistress of Ships is extremely displeased about.