Water is one of the four classical elements, and in the Aristotelian pairing of qualities it is cold and wet, heavier than air and lighter than earth, the element of flow, dissolution and the depths. Symbolically it stands for the emotions, the unconscious, intuition and the receptive, for everything fluid, adaptive and reflective, as water takes the shape of its vessel and mirrors what is above it. The tradition assigns the three water signs, Cancer, Scorpio and Pisces, to it, read as emotional, sensitive and inward.
In the humoral medicine of Hippocrates and Galen water corresponds to phlegm and the phlegmatic temperament, cold and moist, calm and slow. In Western ritual magic water is placed in the west, the quarter of sunset, associated with the chalice or cup and with love, purification and the emotional and psychic faculties. Its attribution to the tarot suit of Cups belongs to the Golden Dawn system of 1888. Water's constant symbolism is of feeling, cleansing, the subconscious and the mysteries of the deep.
Sources The Presocratic PhilosophersG. S. Kirk, J. E. Raven and M. Schofield with The Golden Dawn: The Original Account of the Teachings, Rites, and Ceremonies of the Hermetic Order of the Golden Dawn, 6th editionIsrael Regardie