If you had a bucket of water the size of the Sun and you poured it on it, would you turn it off?

I present to you the Sun and the planets.

Assuming that you managed to make the cube in question, of an anti-solar material (non-existent) and in impossible quantities, and fill it with water, which you would have to bring from other solar systems because in ours there is not enough for the experiment, the water it would evaporate before touching the sun. The safest thing is that the hydrogen and oxygen of the water will separate with the high temperatures and that they will join again causing great explosions. You would be like who says, adding fuel to the fire although water is not firewood, but the Sun is not fire either.

The Sun is an object in a plasma state. It is not fire, nor is its energy generated like that of fire, nor can it be quenched like fire.