A Water nymph (Naiad or Nereid) is a female spirit of the sea in Greek mythology, typically associated with a particular small body of water (e.g. creeks, lakes, rivers). 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 and Persephone. They have no tolerance for campers littering.
Percy Jackson sees two water nymphs waving at him from the bottom of the lake at his first visit at Camp Half-Blood. Percy waves back, but Annabeth Chase comments, "Don't encourage them. Naiads are terrible flirts." They looked like teenagers with long, wavy brown hair, shimmering green T-shirts, and blue jeans.
The water nymphs push Percy, Zoë Nightshade, Thalia Grace, and Grover Underwood up the Colorado River towards the Hoover Dam in a row boat. One of the nymphs playfully splashes Zoë in the face with water, but Zoë does not think it is funny. Zoë says that the naiads have "never forgiven her," but refuses to tell Percy what she means by it.
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 Hercules had done; 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 is startled to see a Naiad out of the water, but she tells him they can act like humans if they want to. Percy feels bad for her and decides not to use her river, and in return she reminds 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 the naiad told him and used this to summon the power of the ocean, which in turn caused an uncontrollable earthquake that launched Percy out of the volcano onto the shore of Ogygia.
Leo, Jason, and Piper are warned by a water nymph that her "crazy cousins" are rampaging through the forest.
- Haliae: Nymphs of the sea, sands and rocky shores. The fifty 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).