Dumping a bucket of water proportionate to the Sun on the Sun would have no significant impact or consequences. The Sun is an incredibly massive and hot star, while a bucket of water, even if scaled proportionately, would be negligible in comparison.
To put things into perspective, the mass of the Sun is about 330,000 times greater than the Earth's mass. Even if you were to scale down the Sun and the bucket of water proportionately, the amount of water in the bucket would still be an infinitesimally small fraction of the Sun's mass.
Additionally, the Sun's surface temperature is around 5,500 degrees Celsius (9,932 degrees Fahrenheit), which is far hotter than the boiling point of water. Thus, the water in the bucket would instantaneously vaporize upon contact with the Sun's incredibly hot surface.
In summary, dumping a bucket of water proportionate to the Sun on the Sun would have no measurable effect. The Sun's enormous mass and intense heat dwarf the water's mass and would cause it to rapidly evaporate without causing any noticeable change to the Sun.