The Water-Nymphs are those creatures who guard the waters. They are known to be playful and cause mischief. The Naiads are the Water-Nymphs who guard streams and fountains of fresh water.

Hylas was Hercules' attendant and well loved by him. As he went to draw fresh water for their meal one day, the nymphs spotted him and immediately fell enchanted. As Hylas outstretched his hand to fill his pitcher, the nymphs seized him and pulled him to the bottom of the dark water. Although Hercules heard his far off cries from under the water, he instead searched the land for his friend Hylas in vain, and was not able to save him.
While grieving the loss of his wife, Orpheus was enticed by the Thracian maidens. He repulsed their advances. In their anger of rejection, they threw weapons at him. But so sweet was his music that the weapons had no effect. This enraged the maidens even more. They drowned out his music with their own screaming voices and hurled their weapons once again at him, this time mortally wounding him. The maniacal maidens tore his limbs off and cast his head and lyre into the river Hebrus. Down the river they all floated until the Muses gathered his body parts and buried him at Libertra. Jupiter placed Orpheus' lyre in the stars where it remains today.

'A Naiade'
by John William Waterhouse
from Sabrina Fair by Milton

Wherewith she sits on diamond rocks,
Sleeking her soft, alluring locks,
By all the nymphs that nightly dance
Upon they streams with wily glance;
Rise, rise and heave thy rosy head
From thy coral-paven bed,
And Bridle in they headlong wave,
Till thou our summons answered have.
Listen and save.

'A Mermaid'
by John William Waterhouse