Naiads

Naiads, River Nymphs or Water Spirits were the gods and goddesses who lived in the rivers and streams of Narnia.

Biography
They were most commonly found in the Great River of Narnia. They are human-like in appearance, and are spiritually tied to the waters of Narnia. No physical description is given, but in illustrations by Pauline Baynes, they are depicted as pretty women dressed in gowns of aquatic colors, most commonly in blue, and sometimes wearing crowns of rushes around their heads.

Their father and chief was the River god, who had a weedy beard. Other River Gods do exist, and are the male equivilant of Naiads.

At the birth of Narnia, they are described as rising out of the river, along with the River God. They also carried Narnia's first female monarch Queen Helen's train at her coronation.

When the White Witch conquered Narnia, she placed the entire land under a spell of constant winter, thus freezing all the water. It seems most likely the Naiads spent most of the Age of Winter frozen, unable to do anything, until Aslan freed them. During the Winter Revolution, they are mentioned among Aslan's followers at the Stone Table, fighting alongside their fellow Narnians against the White Witch.

When the Telmarines conquered Narnia, thus beginning the Telmarine Age of Narnia, the Naiads and the River Gods went into a form of deep sleep. While they slept, the Telmarines built a large bridge over the river, which inadvertently kept the River God, and probably the rest of the Naiads as well, imprisoned.

Eventually, they fought against the Telmarines when they were awoken by Aslan to help fight against the Telmarine army, thus beginning the reign of Caspian X.

Film Adaption
Despite them being mentioned in the books, they were not depicted in the first two films, except for the River God who appeared in Prince Caspian (Disney movie), when he defeated the Telmarine army. Finally, though, the Naiads appeared in the third film, The Chronicles of Narnia: The Voyage of the Dawn Treader (film), as replacements for the Merpeople.

In the film, they are portrayed in a similiar manner to the Dryads, in the respect that they are non-physical beings that use their element as their bodies, which in this case would be water.