For the ancient Greeks, Ocean was imagined as a band which circumscribed the world like a gigantic river. This idea is deeply rooted in the intellectual model of the world which stretches back to the time of Homer. The Greeks did not know with any certainty that there was indeed an ocean which bounded the continents of Europe, Asia, and Africa, but the belief that it did so bound the land was an essential part of their picture of the world.
But the Greek concept of Ocean is so much more than the idea of the world being surrounded by a massive body of water, beyond which further travel was impossible to imagine or achieve. As the entity which bounded their world, it was understood to contribute to the world those things which only the limit of their reality could contribute. To Homer the ‘generation of all’ is the river Okeanos. R. B. Onians tells us that the river ‘surrounds the earth’, and is ‘associated with ‘mother Tethys.  Then he makes an interesting point about the usage of ‘genesis’ by Homer – he suggests that ‘genesis’ suggests ‘the process, or, in this context, the substance rather than the agent of generation. He says ‘that Homer uses it twice of the cosmic river and not elsewhere of gods, men, or animals, which are agents, ‘fathers’, [which] can scarcely be accidental. And indeed it is not accidental. The generative properties of Okeanos are qualitatively different from those possessed by gods, men, or animals, all of which are forms.
Onians continues, saying that Okeanos ‘was believed to be a bond around the earth, apparently of serpent form even as Acheloos, the primal river or water, was conceived as a serpent with human head and horns.’ And that ‘the procreative element in any body was the psuke, which appeared in the form of a serpent’. Thus, Okeanos, ‘as may now be seen, the primal psuke, and thus would be conceived as a serpent in relation to procreative liquid… it can now be explained as the imagined primal cosmic psuke or procreative power, liquid and serpent’. Onians points out the striking similarity between this picture of the world and that found in Mesopotamia, where the earth ‘was encircled by the male element, Apsu, a serpent identified with or in water. With him was another serpent, Tiamat, ‘mother of them all,’’ referencing the Babylonian Epic of Creation. He points out that the Euphrates, thought of as a serpent, was ‘the soul of the land.’ The Mesopotamian concept of the Apsu also embraced the waters of the underworld, so it is easy to understand why among the Greeks, the ‘greatest and most awful oath for the blessed gods’ is, as Onians tells us, ‘by the water of the river of the underworld, the water, the water of Styx proper to the dead.’
It would be easy here to take a detour to explore the relationship between these ideas and the Ionian speculations which have come down to us refracted through the writings of Plato and Aristotle, and the commentators on these writings from the late classical world. There is clearly a connection between Ionian ideas of physis, or ‘as Plato interprets’ genesis, ‘generation’, in their discussions of the primary substance from which all developed. Onians reminds us that Thales argued that the primary substance was ‘water’, and that he thought that the world rested upon water, as well as being surrounded by it. He notes that Aristotle suggests that Thales reason for this view was that ‘he saw that the nourishment of all things is liquid and that the warm is born therefrom and lives thereby and that ‘the seed of all is wet by nature’; and he gives it as the opinion of some people that this view of Thales was the ‘very ancient’ view to be seen in the description of Okeanos and Tethys as ‘fathers of generation’ and in the swearing by the waters of Styx’.
We are dealing here with a very ancient body of ideas, which has strong parallels with ideas which can be found not only in Mesopotamia, but also in countries further east, in Persia and India. Onians also suggests that that the idea of Okeanos ‘is the same belief which underlies the doctrine of the Orphics  and of Pherekydes,  that the first cosmic power was Ophion or Ophioneus’ [pointing out that ophis means ‘serpent’] with his consort described as Okeanis, and that after a struggle with Kronos he dwelt in Okeanos or Ogenos.’  Onians also points out that Philo derives Pherekydes’ teachings from Phoenician sources.’ As already discussed, Okeanos also appears in Homer as the border to the ‘Shield of Achilles’, as at its ‘outermost rim’. The same arrangement holds for Hesiod’s ‘Shield of Herakles.’
The snake-dragon in Mesopotamia is a prominent symbol down to the Hellenistic period. Interestingly it functions as a symbol of various gods, or as a magically protective symbol not associated with any particular deity. Snake gods of Mesopotamia, in particular Nirah, ‘seem to be the only fully animalian, non-anthropomorphic, deities.’  If associated with the idea of the mythological implex of Okeanos, and consequently with the notion of generation (genesis) as opposed to mere fatherhood, one would expect the snake gods to be spoken of differently from other gods. On page 139 of Green & Black’s dictionary of Gods, Demons and Symbols of Ancient Mesopotamia, there is an interesting illustration,  which shows Gudea, prince of Lagash, (neo-Sumerian period), being introduced to the god Enki. This illustration is from Gudea’s own cylinder seal. It shows Enki holding flowing bowls, and the waters emanating from them form a circuit around him. The waters are below his feet, and cross his shoulders. There are three bowls under the seat of his throne; two flowing bowls are his footstool, and another stands behind his throne. Ningišzida has a hand on the bowl held in Enki’s right hand, and there is a lotus symbol emerging from the top of that bowl (a symbol of generation), in addition to the flow of waters. Enki is clearly immersed in the Abzu, which is the ground of Being, as is Okeanos in the Greek world. The circuit of waters is consonant with the idea of Okeanos circumscribing the world. Gudea, Lord of Lagash, is being introduced to the god Enki by his personal god Ningišzida, who holds Gudea’s left hand in his right hand, while he holds a flowing bowl held by Enki, with his left hand. Ningišzida has the serpent symbol, which in Greece is associated with Okeanos, emerging from each of his shoulders.
What does this tableau mean? It is Gudea’s own cylinder seal, so it is relaying an idea which Gudea wants to be understood abroad – at least to those able to understand the language of the image. It is difficult to establish exactly what the image means, but it seems to mean that Gudea is establishing a level of identity with Enki, so that he might be understood to have a connection with the qualities and properties of the Abzu and Enki, both of which are associated with rulership. Reading from right to left, the image is easier to understand: we have Enki, secure in the Abzu, establishing connection with the ground of Being to the god Ningišzida, who holds the hand of the supplicant ruler Gudea. Thus Enki is rewarding Gudea, mediated through Ningišzida, with the qualities of rulership, which have their source in the ground of Being.
The Akkadian name for the ‘vase with streams’ is hegallu, translated as ‘abundance’. This is a symbol which is often found in Mesopotamian iconography. Black and Green describe it as a ‘round-bodied, short-necked, flared-rim jar with streams issuing from its mouth’. The symbol is extensively used in the iconography from Mari, and in other places. The symbolism is often combined with fish swimming in the streams, and sometimes the fish stand in metonomously for the streams, so that the streams are implied rather than shown. The symbolism continued to be used into Achaemenid times. The gods Enki and Ea are often associated with this symbol, as are the various creatures of the Abzu. Black and Green note that the ‘vase with streams’ symbol does not stand for a particular god. Instead they interpret it to be ‘a general attribute of certain divine and semi-divine figures, perhaps
signifying fertility and abundance.’ This interpretation is the best possible in the absence of a proper understanding of the nature of the Abzu and its intellectual context. In fact the flowing vase is a symbol of generation and plenitude, and the latter quality enables the former property.  The Innin temple of Kara-indash at Uruk features relief decoration, in the form of a frieze of deities holding flowing vases. Interestingly they are recessed into the wall, so that they are in the wall, and do not project beyond the limit of the building. The whole design is a symbolic celebration of the idea of the properties of the limit. The bricks which separate the recessed figures are surmounted by the image of the flowing waters. These repeated symbols stand on stele or or kudduru  shaped objects which do stand proud of the wall (as do the symbols of the flowing waters). These objects are regularly used to signify ‘mountain’ (Shamash is often depicted rising above these, with the rays of the sun emanating from his shoulders). The symbol is usually referred to as ‘kur’, which can also mean a road or way. There is an illustration of two of these figures in Oates Babylon.  The representations of the symbol for mountain are reduplicated on the body of the figure on the right. Mountain may also be used as symbol of extreme height (the gods, if they are anywhere, are often supposed to be accessible through high places), in which case this reduplicates the idea of passing beyond a limit (in this case passing beyond the Abzu and coming into the world of existence). In Greece the idea of soul (psuke) is particularly associated with the upper body and the head, and I suggest that some such concept is indicated here. The symbols on the body of the left-hand figure may represent water in the form of wavy lines. Both figures have the upper parts of their bodies bisected by a line of brick parallel with the limit of the wall.
It is the establishment of a connection with the Abzu, which enables rulership. Without this connection, the rulership of the king is not legitimate. Connection with Okeanos is a close parallel of this form of legitimation: as the source of generation, Okeanos makes generation possible. Both Abzu and Okeanos represent the limit of reality, the point of division between the secular world and the divine world. Connection with the ground of Being, identified with this limit, is a connection with the world which is beyond this, a place conceived of as enshrining perfection and greatness. It is also therefore transcendent of this world, and in a sense contains the secular universe, though not necessarily in the same form.
Casting our minds back to Plato’s description of the Living Animal in the Timaeus, we should recall what he said about ‘soul’, which was that it was woven all through the world. We can look at this from more than one point of view. Soul appears to be a property of generated beings which has connection with the ground of Being, whether that be designated by the Abzu, Okeanos, or some other related term. It is this connection which gives generated forms their existence in the representation of reality which is the secular world. So in that sense, soul is woven through the world. But Plato has another idea in mind: Okeanos is woven through the world in that the earth is permeated by rivers, which also have the property of generating forms. In Mesopotamia, the rivers were considered to be divine, marked by the cuneiform sign DINGIR. A river would be indicated by the signs for DINGIR. ID.  It is likely that the notion that rivers were de facto divine in Mesopotamia derives from the assumption, vital within the intellectual model of the world, that they were connected to the Abzu, through which all things have connection with one another. 
1 Onians, R. B. The Origins of European Thought (about the Body, the Mind, the Soul, the World, Time, and Fate). Cambridge University Press, 1951.
2 Fr. 29 K.
3 B. 4 D.
4 This is of some bearing on Plato’s discussion of the Living Animal in the Timaeus, where time comes into being when the same and the different are placed at an angle to each other.
5 Black and Green, Gods, Demons and Symbols of Ancient Mesopotamia, British Museum Press, 1992. p 166,
6 Op. cit. fig. 115, p139.
7 See Black and Green, Gods, Symbols and Demons of Ancient Mesopotamia. Oxford. p 184.
8 Kudduru is a form of boundary marker.
9 Op. cit. p88, figure 60.
10 These signs were used by both the Assyrians and the Babylonians. The signs are ideograms and are derived from the Sumerian lexicon. Akkadian texts are often laden with words and expressions which are written out in Sumerian textual forms, much as we might use Latin or French expressions in the course of writing or speaking.
11 The waters of the Abzu were understood to be of two kinds: the sweet and the bitter. These are easily understood as fresh water and sea water respectively.