The Benefits of offering Water
If one does not have water to drink, he cannot live long. He will become ugly, sad, weak and stupid. If one does not bathe, he will not be clean and fresh. Therefore, the donors of water will be endowed with longevity, beauty, wealth, strength, wisdom, cleanness, fame and great retinue. They will be free from thirst.
A Story showing the Benefits of offering Water
After the Padumuttara Buddha had attained Parinibbana (the wording used instead of die for Buddha), the pious Buddhists offered pure water by pouring the Bodhi Tree(The kind of tree where Buddha stayed under there and did meditation to get Bodhi Enlightenment).
At that time one man also carried perfumed water in a beautifully decorated pot and offered it to the Bodhi Tree.
In his next existence, that man became a deva at the Tusita celestial realm. He enjoyed celestial pleasures as well as royal pleasure in his many existences. In the Gotama Buddha’s time, he became an arahat named Gandhodakiya Thera.
Source : By Ye' Thu Aung (Buddhism for Beginners)