Example 1: You are in the gym working out. At some point you feel very thirsty and say “I want water”.

You don’t want any particular water or all the water available in the world.

You just want “some of the water” which exists in the world. That “water” would be written in genitive. So, we would say “Я хочу воды”.

Example 2: You are again in the gym and again you feel thirsty, very thirsty.

Then you point to a bottle of water standing by my side and I ask you “What do you want? This?” and you answer “Yes, I want the water”. In this case, we know which water you’re talking about.

You don’t simply want water (which would be written in genitive). You want “the water”, that water you’re pointing at. In this example, we would use accusative and say “Я хочу воду”.