Water Nymph

A Water nymph (Naiad or Nereid ) is a female spirit in Greek mythology, typically associated with a particular small body of water (eg. creek, lakes, etc.). Other nymphs, always in the shape of young maidens or 'teenagers', were part of the retinue of a god, such as Dionysus, Hermes, Pan, or a goddess, generally Artemis. They have no tolerance for campers littering.

The Lightning Thief
Percy sees two water nymphs waving at him from the bottom of the lake on his first visit at Camp Half-Blood. Percy waves back, but Annabeth comments, "Don't encourage them. Naiads are terrible flirts." They looked like teenagers having long, wavy brown hair, shimmering green T-shirts, and blue jeans.

The Titan's Curse
Water nymphs pushed Percy, Zoë, Thalia, and Grover up the Colorado River towards the Hoover Dam in a row boat. One of the nymphs splashed Zoë in the face playfully, but Zoë didn't think it was funny. Zoë says that the naiads have "never forgiven her," but refuses to tell Percy what she means.

The Battle of the Labyrinth
A naiad is waiting for Percy at a river when he agrees to clean out the stables of carnivorous horses. It was also one of the tasks Heracles did; she was wearing a green T-shirt and jeans and has grass braided into her hair. She is scared that Percy will destroy her river like Hercules did (and she knows that if he wants to, he can overpower her). Percy was startled to see a Naiad out of the water, but she told him they can act like humans if they want to. Percy felt bad for her and decided not to use her river, and in return she reminded him that he has the sea inside of him, along with how to summon salt water out of petrified seashells.

When Percy was surrounded by the telekhines in Mount Saint Helens, he remembered what she told him and used this to summon the power of the ocean, which in turn caused an earthquake, which in turn caused an eruption that launched Percy out of the volcano.

The Lost Hero
The naiads hauled Jason, Piper, Leo, Butch, and Annabeth out of the lake when they crash landed. Piper stated they had long black hair and glowing yellow eyes.

The Demigod Diaries
Leo, Jason, and Piper are warned by a water nymph that the her "crazy cousins" are rampaging through the forest.

Species of Water Nymphs

 * Haliae: Nymphs of the sea, sands and rocky shores. The 50 Nereids were numbered amongst them.
 * Nereids: Nymphs of the sea, though they are basically equated with the Haliae
 * Leimenids: Nymphs of water-meadows
 * Naiads: Nymphs of the sources of fresh water: springs, fountains, streams, rivers, lakes etc.
 * Heleionomae: Naiads of fresh-water marshes and wetlands.
 * Krenaeae: Naiads of wells and fountains.
 * Limnatides: Naiads of lakes.
 * Pegaeae: Naiads of springs.
 * Potameides: Naiads of rivers and streams.
 * Oceanids: Nymphs presiding over the sources of fresh water, both earthly (springs, rivers,...) and heavenly (moist breezes, clouds,...).
 * Nephelae: Nymphs of the clouds (often counted amongst the Oceanids).