Xavier Remington 
A Nereid is a Sea Nymph which is apart of the greater group of Water Nymphs. Elemental nature spirits who locally personify aspects of nature, and perform various tasks to maintain the planet. Nereid’s are lower level ocean deities who may take orders from higher level sea Gods or Goddesses along with following their free spirit, and hearts as they dwell among the waves, fauna, and flora. Sea Nymphs are responsible for the region of the ocean that is theirs to care for. Water Nymphs will often cooperate with other nature nymphs along with directing Sea Fairies to perform various tasks to maintain the balance of nature. They are the mysterious reason that the oceans survive despite the toxic pollutants humankind has spewed forth within Goddess Gaia’s once pristine waters. Unfortunately there is only so much they can do without violating the Supernatural Secrecy Pact. For instance you can’t have a catastrophic oil spill, and amazingly all life survives along with the water clearing up instantly!
A Nereid is generally in what’s considered a non-corporeal form from our perspective since they’re out of phase with our reality most of time. Hence the lack of Sea Nymph sightings. Although in that form they exist physically in the Paradise Plane aka an Earthly Afterlife for various supernatural beings. In this form all varieties of sea life can see them, and feel their paranormal life force. Sea Nymphs often act as guides, and guardian angels for Mermaids as well. Nereid’s will also phase into our reality to physically interact with the water, wildlife, and our entire world. Sometimes they save humans from Sirens, aka corrupted Nereid’s, and other entities of evil. On occasion they’ve been known to go ashore, and briefly interact in human society. That beyond bewitching beauty at that beach resort was most likely a Sea Nymph. On extremely rare occasions they fall in love with a human, and create a Demi-Nereid.
Are there witches that call upon Nymphs for power? Do all Nereids manage the same amount of the ocean? What are sea faeries? Do Demi-Nereids have Enhanced Strength speed agility etc? Are all Demi-Nereid Female? Do Demi-Nereid have faunapathic abilities? Do Demi-Nereid have hypnotic abilities?
Witches just starting out often call upon lower local deities. Any witch could call upon Nymphs, or Fairies if they sense them in vicinity. The power might be more quickly accessible than calling upon a higher dimensional Goddess. Nereid’s management area is usually based more on the concentration of life than actual square miles. Sea Faeries manage smaller concentrations of life within a Nereid’s zone.