I'm assuming it is like you're telling someone your drinking a specific water when you say, "Bevo l'acqua." I drink the water. So, saying "I drink water" is more general. Meaning, I just drink water so you'll just say, "Bevo acqua."
Why is an article not used here? I'm just beginning so I don't know much at all, but I had just assumed that you needed a 'un, una, il, le' before every noun. I'm guessing from this that that is not true?