Water is a funny element. Its ability to flow makes it a back and forth, being both healing and destructive. World mythology reflects that in its handling of the spirits that live within it.
Naiads were the spirits associated with fresh water in the Greek mythos. They were seperate entities from those found in saltwater, or those associated with specific seas. While this class of spirits were credited with healing abilities, they also were known to destroy men who angered them or led them to be jealous.
Morgens are another violent type faerie or faerie-creature. Like their brethen in Greece, the mogren are known to be attractive women who sing men into the sea-where they drown them. Some myths claim that morgen were taken from the sea as infants but they eventually returned.
On the truly dark end of the spectrum, the nuckelavee are a type of aquatic centuar, known for their single eye and lack of skin. The nuckelavee are exceptionally aggressive and destructive, known to destroy both crops and people when angered. The nuckalevee may be considered to be in the elven family, and are seen as being so strong that the only thing capable of controlling their actions is Mither o’ the Sea. However, it is said that he also hates running water and burning kelp.