I had an interesting talk with a friend about the ring and the circle of Zen. He said that the ring means the union of a couple, whose thoughts and behaviors are dependent on each other, they are still individuals, but are not two anymore. If you are not married, the ring at least means the union of your own mind and body, your own thought and behavior.
The circle of Zen started from a Zen master HuiZhong (677－775) in Tang Dynasty. One time, a monk came to his hall, he drew a circle in the air and wrote a character “say” in the center of the circle, the monk said nothing.
According to his teaching, the circle implies the essence of Zen, when he is drawing the circle; he is expressing the essence of Zen. And later, some Zen masters drew the circle on the paper with brush and ink. When they are drawing, they are expressing the essence of Zen. After the circle is drawn down on the paper, it becomes a symbol of essence of Zen.
I recommend you to draw the circle in the water; it is the same way to express the essence of Zen. In these continuing drawing moments, you are experiencing the union of form and emptiness, you and world, individual and all, existence and Buddha nature…the now, past and future are unified in the every moment now.
It is wonderful that the ring and the circle of Zen have the common significance; either the ring or the circle will remind us of the essence of Zen :)