What if we pour a gigantic bucket of water on the sun?

We really know that the sun will extinguish one day Maybe too hard to answer for anybody but an astronomer with some special software, as nothing like this happens in nature. A star makes a lot of carbon before it makes any oxygen, and here the oxygen is supplied first.

The sun involves a special type of fire that is able to ‘burn’ water, and so it will just get hotter, and six times brighter.

Water is 89 per cent oxygen by mass. And the sun’s overall density is 1.4 times that of water. So if you have a volume of water to the volume of the sun, it will have 1/1.4 = 0.71 times the mass of the sun, and this mass will be .71*.89 = 63 per cent of a solar mass of oxygen and 8 per cent of a solar mass of hydrogen. The sun itself is 0.74 solar masses of hydrogen and 0.24 solar masses of helium.

So you end up with a 1.7 solar mass star with composition 48 per cent hydrogen, 37 per cent oxygen, and 14 per cent helium (with 1 per cent heavier elements). Now, will such a star burn? Yes, but not with the type of fusion the sun uses… It will be bluish-white with more UV. Along with six times the heat input the Earth’s biosphere will be fried, and oceans probably boil.