Water, one of the four western classical elements (the others being earth, air and fire), symbolizes passivity, adaptability, purity, fertility, healing and cleansing. It is associated with the emotional-intuitive or feminine aspect (just as fire is the masculine).
According to Kurt Seligmann's The History of Magic and the Occult (1948), the year was traditionally divided in two by ancient peoples: spring-summer was the male season of the sun, warmth and dryness while autumn-winter was the female season of the moon, coolness and moisture because it is the wet season for most of the world.
Water is connected to the moon scientifically, as well, as the moon's gravitational pull affects tidal flow.
In Isaac Newton's translation of the Hermetic Emerald Tablet, he renders the elements as Sun (fire), Moon (water), Wind (air) and Earth - the four elements 'above and below' that come together for the One thing (Life).
In the Tao Te Ching we are told to 'know the masculine but keep to the feminine.' Bruce Lee quotes an ancient Oriental proverb when he says "Be water my friend, be water," which is to say be flexible, adaptable in mind and body.
Its cleansing properties meant that it was seen as a means of purification, whether in the form of individual baptism or global deluge (e.g. Noah's or Deucalion's Flood, Manu's, etc.), both of which are universal ideas. Similarly its opposite (fire) was considered equally 'cleansing,' as can be seen in The Book of Revelation.
For this reason it was also considered by the alchemists to be a "universal solvent," a notion modern science echoes precisely. There are two forms of alchemical water, says Socrates, but both are described as 'fire' and 'water' in a single substance. (See: C.G. Jung, Psychology and Alchemy.)
There is a very old parable, ultimately of Oriental origin but it appears in the Book of Matthew, which describes Enlightenment as the crossing of a river.