I would imagine that it is because of the depths behind the reflections, especially the reflection of self and the world as we know it, that water has become symbolic of the unconscious - or at least one of the reasons for the symbolism.
". . . myth is perhaps the most important psychological and cultural construct of our time. It is not only that the concept of myth has degenerated in popular parlance into something synonymous with falsehood. Or that myth, as it has been said, is someone else's religion. It is that, in a culture committed to the world of matter, access to the invisible world - which myth makes possible, along with its two chief instruments, metaphor and symbol - has never been more critical in allowing some balance of the spirit." (Hollis, Mythologems, p. 8)As I teach here in Changzhou, I make sure that one of my first lessons include the topic of metaphor. In cross-cultural situations when learning/teaching a second language, the images that words evoke are vital to successful communication. It only makes sense then that images, both verbal and symbolic, are just as vital when trying to enter into the realm of the unconscious, the realm of the spirit and the soul. The language of metaphor and symbol are my tools for the work I do in trying to uncover the hidden and buried aspects of myself. The work is not much different that peeling away the layers of an onion wherein after the peeling of a layer, the onion is still an onion, but one is closer to the essence of the onion.
As I descend, layer upon layer, a bit more of myself is exposed to both myself and others around me. It is as if I am being stripped of artifice and masks. When the last layer is peeled away, one could conceivably say that nothing is left - or, one could say that the self has somehow expanded to include everything. In Jungian terms, this is close to what one means when we say that the self morphs into the SELF - where there is no separation, where there is only the ONE - all that is and isn't held together - some would say this is God.