Snake in the grass

i) One reason we need to be circumspect about using comparative mythology to decode Biblical symbolism is that the same items can have varied significance in world mythology. So there's the risk of sample selection bias. Of superimposing an alien gloss onto the text. 

ii) It's possible that the symbolic import of some natural elements is a cultural universal. But it's hard to make confident generalizations given the vast scope of the topic over time and place. The available evidence is unmanageably large, and even then, that's only scratching the surface. 

iii) Some natural elements, because they have different functions, inevitably give rise to different or divergent symbolic meanings. Take fire. That can be used for heating and cooking. Keeping predators at bay. Purifying ores (metallurgy). But, of course, it can also be destructive. 

Likewise, take water. Too much water may be fatal (drowning). Too little water may be fatal (dying of thirst–or dying of hunger from famine due to drought). 

Water is used for so  many different things. Washing, cooking, drinking, &c. 

That's why scholars disagree on the significance of baptism. It's often thought to represent cleansing. But some scholars think it represents amniotic fluid, while Meredith Kline thought it represents deliverance from death by drowning. 

iv) Fauna, flora, and landscape have symbolic significance in many different cultures. But, of course, different cultures often have different faun, flora, and landscapes. Had Gen 3 been revealed in a culture with different animals, the Tempter might have been named Fox rather than Snake–since the fox is a trickster animal in some folkloric traditions.

v) Moreover, the same natural elements can have variable symbolic significance. For instance, many different symbolic roles and properties are attributed to snakes. 

vi) To take another comparison, consider rivers. I suspect temperate rivers have a generally benign symbolism, but tropical rivers might well have an ambivalent or ominous connotations. For instance, the Nile has hippos and crocodiles. That makes the Nile river hazardous to humans. Likewise, many hidden dangers lurk in the Amazon river, viz., the Piranha, tiger fish, anaconda, electric eel, stingray, Bull shark, black caiman. 

vii) To take another comparison, some mythologies view fabled islands as heaven on earth (e.g. Dilmun, the Isles of the Blessed). Yet an island which appears to be a tropical paradise can be very menacing beneath the balmy surface. The sandy beach main contain deadly cone snails. The waters may contain sharks, stonefish, box jellyfish, &c. The scenic jungle may contain venomous snakes, giant pythons, or poisonous spiders. The island would have very different associations to a native than a passerby. 

vii) By the same token, mythological utopias like Dilmun and the Garden of the Hesperides are both "Edenic" or paradisiacal, yet these gardens for the gods, not humans. 

viii) Now I'd like to quote from a standard reference work to illustrate the diverse ways in which world mythology interprets "Edenic" motifs:

As the center of the world, linking heaven and earth and anchoring the cardinal directions, the mountain often functions as an axis mundi–the centerpost of the world…One of the most important such mountains is Mount Meru, or Sumeru, the mythical mountain that has "centered" the world of the majority of Asians–Hindu, Buddhist, and Jain. "Mountains," L. Jones, ed. Encyclopedia of Religion (Macmillan, 2nd ed., 2005), 9:6212a. 

Mountains are the source not only of nourishing waters but also of rains and lightning. Storm gods are often associated with mountains: Zeus, Rudra/Siva, Baal Hadad of Ugarit, Catiquilla of the Inca, and many more.  

Mountains, the source of the waters of life, are also seen as the abode of the dead…Among the Shoshoni of the Wyoming, for instance, the Teton Mountains were seen primarily as the dangerous place of the dead. The Comanche and Arapaho, who practiced hill burial, held similar beliefs. "Mountains," ibid., 9:6214b. 

Above all, the influence of the desert environment appeared in the way in which, in the West, the garden was seen as an oasis, in stark contrast to the barren wastes outside…Confusingly, there was another more puritanical tradition in which the roles were reversed, and the garden, with its luxury, was condemned as the scene of temptation, while the wilderness was celebrated as the true paradise. "Gardens: An Overview," Ibid. 5:3277a. 

In China and Japan, both the awesome mountains and the streams that issued from them were thought to be possessed by spirits, and they were considered to be alive like plants animals and human beings themselves…To the Buddhist, the garden furnished a lesson on time. The flowers opened and withered within a month. The seasons revolved. But stone decayed on a far longer time scale that turned the present into a moving infinity. The symbolism was as varied and extensible as the clouds that gathered around the mountain peaks. Ibid. 5:3277b.  

The garden contained both friendly and unfriendly spirits. But threatening spirits were not persecuted as they might have been in the West: they were either left undisturbed (for example, by not digging the ground too deeply) or frustrated (as in the case of the demons who traveled in straight lines, who were thwarted through the construction of zigzag bridges). Ibid, 5:3277b.   

Real-life peasants and laborers, on the other hand, with families to feed, know that in temperate latitudes the skills involved in planning and maintaining a subsistence garden are greater than those called for in a recreational or cosmic garden because most of the edible plants are annuals….Things are different in parts of the tropics where three crops may be harvested in a year and the division between extensive fields and intensive gardens breaks down. There, the subsistence garden may assume an idealized form. Ibid. 5:3278b.   

Dilmun [is] a place that is pure, clean, and bright, a land of the living who do not know sickness, violence, or aging…a garden with fruit trees, edible plants, and green meadows. Dilmun is a garden of the gods, not for humans, although one learns that Ziusundra, the Sumerian Noah, as exceptionally admitted to the divine paradise. "Paradise," ibid. 10:6981b. 

Crossing the river at the time of death, as part of the journey to another world, is a common part of the symbolic passage that people have seen as part of one's journey after death. In the Epic of Gilgamesh, the hero encounters a boatman who ferries him across the waters of death, as he seeks the source of immortality. The river Styx of Greek mythology is a well-known as the chief river of Hades. 
The dry riverbed of Sainokawara is said to be the destination of dead children. In the Buddhist tradition, nirvana is referred to as the "far shore". "Rivers," Ibid. 11:7862b-63a.  

To Hindus, the Ganges is the archetype of all sacred waters; she is a goddess, Mother Ganga, representative of the life-giving maternal waters of the ancient Vedic hymns…According  to Hindu belief, the Ganges purifies all she touches…Pilgrims go to these places to bathe in the Ganges, to drink her water, to worship the river, and to chant her holy name. Especially in Banaras, many come to cremate their kin, to deposit the ash of the dead in the river, or to perform religious rites for their ancestors. Some come to spend their last days on the banks for the river, to die there and thus to "cross over" the ocean of birth and death….All who come to the Ganges come in the firm belief that bathing in this river, even the mere sight of Mother Ganga, will cleanse them of their sins… "Ganges River," ibid. 5:3274. 

Clearly, it's unreliable to assume that ancient Near Eastern mythology encodes culturally universal intuitions regarding the emblematic significance of a river, mountain, or garden paradise. We can't just default to that frame of reference as the presumptive background material for decoding the symbolism of Eden.