Heavenly Waters is described in its own entry as a modern label for a family of watery constellations rather than a single figure. What that description does not explain is where the watery theme came from in the first place, since the Greek myths attached to its members, a river, a dolphin, a fish, a ship, tell unconnected stories that share nothing except where they happen to sit.
The theme predates the myths. This stretch of sky, running roughly from Aquarius and Capricornus through Piscis Austrinus, Eridanus, Delphinus, Equuleus and the ship Argo, was already read as a sea in Babylonian sky-lore, inherited by Greek astronomers along with the zodiac itself, because the Sun crossed this exact region during the rainy season of the ancient Near East. A culture whose year turned on when the rains came would naturally mark the part of the sky the Sun rode through at that time, and the Greeks, receiving both the zodiac and its neighbouring watery quarter from the same older tradition, supplied individual stories for individual stars without inventing the region's theme.
Ptolemy's own Almagest never names this family as a formal unit at all; his forty-eight are sorted only into zodiacal, northern and southern lists, and Heavenly Waters is a convenience used by later historians of astronomy, Richard Hinckley Allen among them, to describe a real pattern in what got grouped where rather than a boundary the ancient text itself drew.
Read this way, the family is less a set of stories about water and more a fossil of an agricultural calendar that outlived the civilisation that built it.