The reference to Neptune in this line of "The Olympic Swimmer." is most likely intended as a mythological allusion.
When refering to Neptune, the author of "The Olympic Swimmer" is alluding the god of freshwater and the sea in Roman religion. He is the counterpart of the Greek god Poseidon. Neptune was the brother of Jupiter and Pluto; the brothers presided over the realms of Heaven, the earthly world, and the Underworld. Salacia was his wife.
