Ok what we know about Cupid - some of which we read while reading Ovid last winter -
Cupid is winged, allegedly, because lovers are flighty and likely to change their minds, and boyish because love is irrational. His symbols are the arrow and torch, "because love wounds and inflames the heart." Cupid is also sometimes depicted blindfolded and described as blind, not so much in the sense of sightless—since the sight of the beloved can be a spur to love
Cupid carries two kinds of arrows, one with a sharp golden point, and the other with a blunt tip of lead. A person wounded by the golden arrow is filled with uncontrollable desire, but the one struck by the lead feels aversion and desires only to flee. - In
Metamorphous we read how Apollo taunts Cupid as the lesser archer, Cupid shoots him with the golden arrow, but strikes the object of his desire, the nymph Daphne, with the lead. Trapped by Apollo's unwanted advances, Daphne prays to her father, the river god Peneus, who turns her into a laurel, the tree sacred to Apollo.
In Ovid’s
Metamorphoses we also have the story how Cupid accidentally wounded Venus with one of his arrows when he leaned in for a kiss, he causes the goddess’s tragic infatuation with Adonis. We also have Venus eager to demonstrate love’s power over the underworld. At her command, Cupid fires an arrow at Pluto who becomes enamored of Ceres’ daughter, Proserpine.
Cupid is often shown riding a dolphin. One interpretation is the image represent the soul's journey, originally associated with Dionysian religion. Dolphins were often portrayed in antiquity as friendly to humans, and the dolphin itself could represent affection.
Cupid's triumph over mankind is signified by his driving a chariot pulled by lions
Cupid sleeping became a symbol of absent or languishing love in Renaissance poetry and art, including a Sleeping Cupid (1496) by Michelangelo that is now lost. The image of Cupid or Amore sleeping represents the indolence of Love in the lap of Idleness.
I cannot find anything in the Cupid Psyche myth where Cupid is sleeping by the water with his bow and arrow by his side however, there are several paintings of that scene.
In his play,
Midsummer Night's Dream there are all sorts of children type angles and there is a time they are often shown by a pond as well as asleep in the forest or in a large bed. The play does not specify that any of them represent cupid but given the nature of the story it could be and therefore in this Sonnet Shakespeare could be carrying that thought describing cupid and water both heated by fire or cooling love.
Now here is a story I did not know and I find fascinating - According to Cicero and Pausanias, Cupid had a brother, born after him to the same parents: Venus and Mars. Anteros is sometimes viewed as Cupid’s enemy, representing spiritual rather than carnal love, and featured struggling for victory in a contest over a palm. Anteros is symbolic of Reciprocal or Virtuous love, and his tussling with Cupid is associated with the celebration of marriage, which required both Loves to prove fruitful and fulfilling.