The Naiads.

While researching the merfolk I came across many different sea myth creatures (a lot of which I knew about already), so I thought I would explore the water world a bit more ...

The naiads (Ancient Greek: Ναϊάδες) are where a fresh water nymph, living in rivers, fountains, lakes, springs and marshes. They were immortal (although some sources say they where on long lived), minor divinities who were invited to attend the assemblies of the gods on Mount Olympus.

 The naiads weren't bound to their rivers or lakes, the Greeks believed that all the water of the world was connected by deep rivers under the earth, so although their spirits where bound to their springs they could move around, but if their water source dried up then they would die too.

They, along with Artemis, where seen as the divine nurses of the young, and the protectors of girls and maidens, overseeing their safe passage into adulthood.They were often the object of archaic local cults, worshipped as essential to humans. Boys and girls at coming-of-age ceremonies dedicated their childish locks to the local naiad of the spring. In places like Lerna their waters' ritual cleansings were credited with magical medical properties. Animals were ritually drowned there.  They where also known for telling the future and often foretelling humans their destiny.

Those who possessed waters with some special property (or which were believed to have some special property), often had proper cults and shrines established in their honour. Examples of these include the Anigrides of Elis whose waters were believed to cure disease, and the Nymphs of Helikon and Delphoi whose waters were thought to bestow poetical and prophetic inspiration respectively. They often gave their names to towns, cities and islands, and as such were most likely regarded as the goddess-protectors of the community's water supply.

The Naiades of the rivers and marshes were attendant-nymphai of their River-God fathers. A few of these Nymphai were called goddesses of their own small tributary river, but for the most part they were rarely mentioned as individuals or received cult status, not like the naiads of springs and fountains. Although they aren't necessarily goddesses they where the nurses of the other gods including Dionysos by the Lamides and Naxian Nymphs, and Hera by the Asterionides. Others were found in the retinues of gods, such as the Naiades Bakkhai in the train of Dionysos, and Artemis' band of Amnisiades.

Although they where seen as calm serine beings, just like water they could be dangerous, Hylas of the Argo's crew was lost when he was taken by naiads fascinated by his beauty. and extremely jealous.

There are different types of naiads and each one had their own names.
 Crinaeae (fountains)
Eleionomae (marshes)
Limnades or Limnatides (lakes)
Pegaeae (springs)
Potameides (rivers)

There are many tales of individual naiads, here are a few: Aba, Bateia, Creusa, Lara, Moria, Orseis, Polyxo and Tiasa.