Naiad

Naiads are a type of Nymph which preside over fountains, wells, springs, streams, brooks, rivers, and marshes. They are further divided into various subclasses: The Naiads are intimately connected to her body of water and their very on existence seems to have depended on it. If a spring dried out, the Naiad presiding over it might die.
 * Crinaeae (fountains)
 * Pegaeae (springs)
 * Eleionomea (marshes)
 * Potameides (rivers)
 * Limnades or Limnatides (lakes)

As they are the spirits of bodies of fresh water, they do not serve directly in Poseidon's court. Charybdis, one of the monsters in the Sea of Monsters made famous by the Odyssey, was once a beautiful naiad as well, although she was turned into a monster because Zeus was angry that she was claiming so much land and turning it into the sea. Many naiads live in the Canoe Lake at Camp Half-Blood.

The Titan's Curse
The Naiads help to push the boats upstream when Percy Jackson asks. While Percy and Zoë Nightshade were in the canoe together, one splashed water at her - she said that they had never forgiven her, maybe because she had become a Huntress instead of staying with the Hesperides.

The Battle of the Labyrinth
"You're not so different from me, demigod. Even when I'm out of the water, the water is within me. It is my life source. I hope you find a way to rescue your friends."

- A Naiad advises Percy Jackson

There was a naiad that lived in the river near the Triple G Ranch. Percy had planned on using the river to clean the flesh-eating horses' stables but she didn't want her river to become polluted again, as it did when Hercules was given the same labor. The naiad was prepared to fight to protect the river even if Percy was the son of Poseidon and he could easily beat her, but Percy didn't want to push her around and refused to fight, surprising the naiad. The naiad reminded Percy of Rachel Elizabeth Dare because she was asking so many questions it felt like she was throwing rocks at him. She wanted to avoid the river pollution, so she taught Percy how to summon seawater to clean away the dung by throwing fossilized seashells onto the ground by claiming that water used to cover everything and the sea follows Percy wherever he goes.

Difference between the Nereids
Their cousins, Nereids, are spirits of the sea and other bodies of salt water. Unlike the Naiads, Nereids serve in Lord Poseidon's court. Also when Nereids leave, they glow green and vanish. When Percy was young, Nereids (Naiads would or could not be there in the sea) would watch him with interest at the beach in Montauk and whenever he was near the ocean.

Trivia

 * Naiad is a moon of Neptune. It was named after this species.
 * They, like Artemis, were seen as the divine nurses of the young and the protectors of girls and maidens.
 * They were said to be the daughters of the Potamoi (gods of the rivers and streams on Earth).