Naiad

They are distinct from river gods, who embodied rivers, and the very ancient spirits that inhabited the still waters of marshes, ponds and lagoon-lakes, such as pre-Mycenaean Lerna in the Argolis.

Naiads were associated with fresh water, as the Oceanids were with saltwater and the Nereids specifically with the Mediterranean, but because the Greeks thought of the world's waters as all one system, which percolated in from the sea in deep cavernous spaces within the earth, there was some overlap. Arethusa, the nymph of a spring, could make her way through subterranean flows from the Peloponnesus, to surface on the island of Sicily.