For the satisfaction of ancestors, we perform tarpan (offering) with water. When we give oblations to the Sun-God, we use water, flower, sandalwood paste and leaf. What is the point of showing a lamp to the sun, the source of light? Hence we offer water.
Sun means brightness. In the five elements, from the space originates air, from the air originates fire, from fire originates water and from water originates earth. This shows that water is the son of brightness. Offering water therefore pleases the Sun-God.