In Nichiren Daishonin’s day (13th century Japan), the first water drawn from the well was offered to the Gohonzon. For this reason, we place fresh water in a cup in front of the Gohonzon first thing in the morning each day. There are two deep meanings behind this practice:
1) The function of water is to wash away filth and impurity. This refers to the function of chanting Nam Myoho Renge Kyo, which lets you extinquish your own bad karma and past slander.
2) The second meaning is that just as water flows from a high place downward to lower ones, the Buddha’s compassion and mercy flows from the heights of the world of Buddhahood downward to equally benefit all life in the lower nine worlds.
Further details about how to offer the water and to care for the Gohonzon and your altar can be found in the book, Basics of Practice.
Other articles on the Buddhist Altar:
Reference: “Daily Care of the Gohonzon: The meaning of offering the water, the evergreens, light and fragrance” Nichiren Shoshu Monthly, November, 1997.