Arethusa is the nymph of a spring of the same name at Syracuse on the island of Sicily. Her spring is located on the small island of Ortygia, just off the coast and connected to the rest of the city by a causeway in antiquity; it is now built into an enclosed pool with high sides, and in the pool papyrus is planted and white ducks are kept. Before it was enclosed it must have poured almost directly into the Sea; even now the beach is just a short distance over the wall.
Arethusa's legend is Greek, as the colony at Syracuse was founded by emigrants from Corinth, Greece in about 734 BCE. In the tale Arethusa was a nymph of Elis (located in the western Peloponnese) and a friend of Artemis, and like Her a virgin huntress who couldn't care less about men. She was extraordinarily beautiful but considered it rather a nuisance; She much preferred the wild woods. One hot day, Arethusa stopped to bathe in the River Alpheus, but as She was so doing the God of the river saw Her and became enamoured of Her. She refused His overtures; and when He wouldn't take no for an answer She fled. He chased Her for some time, but when She eventually tired and feared He would catch Her, She prayed to Artemis to save Her. Artemis did just that: She carried Her off in a mist all the way to Sicily, where She became the spring of Arethusa.* Alpheus, however, was not so easily foiled, and it is said that He journeyed under the sea to Her new spring, where His waters mingled with Hers. It was professed in ancient times that a flower or cup dropped into the Alpheus River in Greece would flow under the ocean and eventually pop up in the spring of Arethusa all that distance away. The River Alpheus does at one point flow into the ground, perhaps explaining the connection the Greeks believed it had with Arethusa's distant spring.
At any rate Arethusa's spring on Ortygia was very close to the Sea; perhaps this connection between the two bodies of water partially inspired the tale as well. Another factor in the story was that it provided a connection to the settlers with their old homeland of Greece. Arethusa may very well be a name given to an already existing nymph of the spring, as there is evidence that Ortygia had been inhabited since Paleolithic times, and a spring that dramatically situated would likely have had its own Goddess for some time. And Ortygia, in case you are confused, is also a name for the tiny Greek island of Delos, considered one of its holiest places, for Artemis and Apollo were born there.
Arethusa was considered a Nereid, or Sea-nymph, one of the fifty daughters of the Sea-god Nereus, and in Italy was known as one of the Sicilian nymphs, who inspired much poetry. She was depicted on coins of Syracuse of the 5th century BCE, and is shown as a wavy-haired woman with a laurel wreath in Her hair, surrounded by circling dolphins.
There are several other springs called Arethusa in Greece, and Arethusa is a name shared with one of the Hesperides, Greek Twilight Goddesses who guarded the tree of golden apples Who lived in the far far west.
Also called: Arethousa.
*These tales of "rescues" through transformation had always struck me as unfair, in that the nymphs have no choice but to trade Their human-like body for safety—for example when Daphne calls on Her father to save Her from Apollo and is changed into a laurel tree, or Arethusa, as above, is transformed from a nymph to a spring—but now it occurs to me that they are really the same thing: Daphne is already both nymph and laurel tree, as Arethusa is both nymph and spring, for that is Their fundamental nature as the spring or tree deified or personified. The legends, in trying to explain things, have gotten things backwards by making Their original natures the end result of the story.