Naiad

Naiads are river and lake spirits, they are the spirits of bodies of fresh water and 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 live in the Canoe Lake at Camp Half-Blood.

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.

The Lightning Thief
In The Lightning Thief, Percy meets a Nereid though he didn't know what she was at the time at The Arch when he battled Chimera. The Chimera poisoned Percy, so he jumped off the Arch in St. Louis in the Mississippi River, which saved his life. Then he sees her and she says, "Go to the beach in Santa Monica." He does and meets her again. There, Percy can see her better too. The Nereid gives him three white pearls, tells him that his fathers court has been watching and leaves him for the last time.

The Titan's Curse
While Percy and Zoë 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
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. 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, surpising 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 use to cover everything and the sea follows Percy where ever he goes.

Difference between the Nereids
The Naiads are fresh water nymphs, unlike the Nereids who are salt water nymphs.

Trivia

 * Naiad is a moon of Neptune. It was named after this species.