Water (excerpt from Understading Buddhism, Venerable Master Chin Kung)

Great Strength Bodhisattva Buddha Amitabha Guan Yin Bodhisattva

Therefore, everything in a way place serves as a teaching aid. Even the offerings are educational. A container of water symbolizes the Dharma. The water is clean symbolizing that our minds need to be as pure as the water. It is calm without a single ripple, symbolizing the tranquility in our hearts. It is to have purity and equality of mind.