Water Nymph

In Greek mythology, a water nymph (aka naiad or Neried) is a female spirit typically associated with a particular small body of water (ex. creek, lakes, etc.). Other nymphs, always in the shape of young nubile maidens, were part of the retinue of a god, such as Dionysus, Hermes, or Pan, or a goddess, generally Artemis.

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 Titans Curse
Water Nymphs pushed Percy, Zoe, Thalia and Grover up the Colorado River towards the Hoover Dam in a rowboat. One of the nymphs splashed Zoe in the face playfully, but Zoe didn't think it was funny.

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