Piscis Austrinus is already described, in its own article, as the parent of the zodiac's two fishes and as drinking the water poured from Aquarius's jar. That drinking posture is not an isolated image: it sits inside a much larger old reading of this entire quarter of sky as a sea.
The cluster running from Aquarius and Capricornus through Piscis Austrinus, Eridanus, Delphinus, Equuleus and the ship Argo shares no boundary and no single myth, only a persistent watery theme that predates Greek astronomy. Babylonian sky-lore, inherited by the Greeks along with the zodiac itself, already read this stretch as a sea, because the Sun crossed it during Mesopotamia's rainy season; Piscis Austrinus's fish, drinking from the jar beside it, is a natural extension of a reading that was already there before either figure had a name.
Modern historians of astronomy label the loose cluster the Heavenly Waters, a convenience term rather than a boundary Ptolemy drew, since his own forty-eight were sorted only into zodiacal, northern and southern lists.
Fomalhaut, the constellation's one bright star and by far the most prominent point of light in this whole watery region, makes Piscis Austrinus the most visible single member of the ancient sea's cluster of otherwise faint figures, a bright mouth in a mostly dim quarter of sky.