First of all, you cannot ‘put out’ or ‘extinguish’ a star. You can only extinguish something that is burning. The source of a star’s energy is the fusion reaction going on in their core. The only way you can stop a star’s nuclear fusion is by depriving it of it’s main fuel – hydrogen. As for your second question, water will only make the star to glow brighter. Water adds mass to the star and also gives some extra hydrogen. So there is no way you can extinguish a star instantly. On the other hand, you can hasten the nuclear reaction inside the core by adding up more mass to the surface of the star. In that case, water will do the job. But if you stay around the sun with a water hose forever, you won’t survive the supernova after that.
