HYDROS was the god of the primordial waters.
In the Orphic Theogonies Hydros (Water), Thesis (Creation) and Mud were the first entities to emerge at the dawn of creation. Mud in turn solidified into Gaia (Earth) who, together with Hydros, produced Khronos (Chronos, Time) and Ananke (Compulsion). This latter pair then crushed the cosmic-egg with their serpentine coils to hatch Phanes (Life) and divide the cosmos into its constituent parts--Heaven, Air, Earth and Sea.
The Orphic Rhapsodies, which evolved from the older Theogonies, discarded the figures the Khronos and Ananke and instead have Phanes born directly of Hydros and Gaia.
The figure of Hydros was derived by the Orphics from Okeanos (Oceanus)--the earth-encircling, fresh-water river which Homer describes as the source of the gods. His consort Thesis was likewise the primordial counterpart of Okeanos' wife Tethys.
FAMILY OF HYDROS
 NONE (emerged at creation) (Orphic Fragments 54 & 57)
CLASSICAL LITERATURE QUOTES
I. THE HOMERIC COSMOGONY
Homer, Iliad 14. 200 ff (trans. Lattimore) (Greek epic C8th B.C.) :
"[Homer portrays Okeanos (Oceanus) and Tethys as the primordial gods of creation :]
The ends of the generous earth on a visit to Okeanos, whence the gods have risen, and Tethys our mother."
[N.B. The Orphics developed this concept into the figures of Hydros and Thesis.]
II. THE ORPHIC COSMOGONY
Orphica, Theogonies Fragment 54 (from Damascius) (trans. West) (Greek hymns C3rd A.D. - C2nd B.C.) :
"Originally there was Hydros (Water), he [Orpheus] says, and Mud, from which Ge (Gaea, the Earth) solidified : he posits these two as first principles, water and earth . . . The one before the two [Thesis], however, he leaves unexpressed, his very silence being an intimation of its ineffable nature. The third principle after the two was engendered by these--Ge (Earth) and Hydros (Water), that is--and was a Drakon (Dragon-Serpent) with extra heads growing upon it of a bull and a lion, and a god's countenance in the middle; it had wings upon its shoulders, and its name was Khronos (Chronos, Unaging Time) and also Herakles. United with it was Ananke (Inevitability, Compulsion), being of the same nature, or Adrastea, incorporeal, her arms extended throughout the universe and touching its extremities. I think this stands for the third principle, occuping the place of essence, only he [Orpheus] made it bisexual [as Phanes] to symbolize the universal generative cause.
And I assume that the theology of the [Orphic] Rhapsodies discarded the two first principles (together with the one before the two, that was left unspoken) [i.e. the Orphics discarded the concepts of Thesis, Khronos and Ananke], and began from this third principle [Phanes] after the two, because this was the first that was expressible and acceptable to human ears. For this is the great Khronos (Unaging Time) that we found in it [the Rhapsodies], the father of Aither (Aether) and Khaos (Chaos). Indeed, in this theology too [the Hieronyman], this Khronos (Time), the serpent has offspring, three in number : moist Aither (Light) (I quote), unbounded Khaos (Air), and as a third, misty Erebos (Darkness) . . .
Among these, he says, Khronos (Time) generated an egg--this tradition too making it generated by Khronos, and born ‘among’ these because it is from these that the third Intelligible triad is produced [Protogonos-Phanes]. What is this triad, then? The egg; the dyad of the two natures inside it (male and female), and the plurality of the various seeds between; and thirdly an incorporeal god with golden wings on his shoulders, bulls' heads growing upon his flanks, and on his head a monstrous serpent, presenting the appearance of all kinds of animal forms . . .
And the third god of the third triad this theology too celebrates as Protogonos (First-Born) [Phanes], and it calls him Zeus the order of all and of the whole world, wherefore he is also called Pan (All). So much this second genealogy supplies concerning the Intelligible principles."
Orphica, Theogonies Fragment 57 (from Athenogoras) :
"The gods, as they [the Greeks] say, did not exist from the beginning, but each of them was born just as we are born. And this is agreed by them all, Homer saying ‘Okeanos the genesis of the gods, and mother Tethys [Thesis],’ and Orpheus--who was the original inventor of the gods' names and recounted their births and said what they have all done, and who enjoys some credit among them as a true theologian, and is generally followed by Homer, above all about the gods--also making their first genesis from water : ‘Okeanos (Oceanus), who is the genesis of the all.’
For Hydros (Water) was according to him the origin of everything, and from Hydros (the Water) Mud formed [primordial Gaia], and from the pair of them a living creature was generated with an extra head growing upon it of a lion, and another of a bull, and in the middle of them a god's countenance; its name was Herakles and Khronos (Chronos, Time). This Herakles generated a huge egg [which forms the earth, sea and sky]."
Apollonius Rhodius, Argonautica 1. 503 ff (trans. Rieu) (Greek epic C3rd B.C.) :
"He [Orpheus] sang of . . . How, in the beginning, Ophion [Ouranos the Sky?] and Eurynome [Gaia the Earth?], daughter of Okeanos [Hydros the Waters?], governed the world from snow-clad Olympos; how they were forcibly supplanted, Ophion by Kronos (Cronus), Eurynome by Rhea; of their fall into the waters of Okeanos."
Nonnus, Dionysiaca 23. 280 ff (trans. Rouse) (Greek epic C5th A.D.) :
"[Nonnus portrays Okeanos (Oceanus) and Tethys as the primordial gods of creation, i.e. counterparts of the Orphic Hydros and Thesis :] Tethys! Agemate and bedmate of Okeanos), ancient as the world, nurse of commingled waters, selfborn, loving mother of children."
Nonnus, Dionysiaca 41. 155 ff :
"Aion (Time), his [Okeanos'] coeval [of the same age]."
- Homer, The Iliad - Greek Epic C8th B.C.
- Apollonius Rhodius, The Argonautica - Greek Epic C3rd B.C.
- Orphica, Theogonies Fragments - Greek Hymns C3rd B.C. - C2nd A.D.
- Nonnus, Dionysiaca - Greek Epic C5th A.D.