The latter two deserve a bit more explanation. Before her marriage to Hēraklēs, Hebe, daughter of Zeus and Hera, acted as cup bearer to the Gods. When she wedded, Zeus cast his eyes about and took a shining to a young boy from Troy: Ganymēdēs. There are various versions of the myth. In one, Zeus transforms himself into an eagle and swoops up the boy, carrying him off to Olympus. In another, he pays Ganymēdēs' father Tros with 'horses beautiful and strong enough to carry the Gods themselves'. In all versions, Ganymēdēs is made immortal and his father is comforted by that knowledge. All Gods were overjoyed to have the young man serve them, except for Hera, who saw a rival for her husband's affection in the boy. It was Zeus who later put Ganymēdēs into the sky as the constellation Aquarius.
The other option is that the constellation represents Deukalion. He and his wife Pyrrha, daughter of Pandôra, recreated humanity by tossing stones behind them as they walked, after Zeus flooded the Earth and killed humanity for its hubris. You can read the full story here, as I have blogged about it before. According to myth, Zeus put Deukalion into the sky to honor his contribution to humanity. Sadly, Pyrrha was not awarded this honor.
Whomever the constellation is supposed to represent, the constellation is very old and--as it's there in every horoscope, ever--very powerful. The Aquarius constellation is visible at latitudes between +65° and −90°. It is best visible at 21:00 (9 p.m.) during the month of October.