When to use accusative article
Why do we use an article on some nouns and not on others? Is there a general rule? "Ich mag den Apfel" but we say"Ich mag Wasser"?
You don't need to use an article with mass nouns like water(doesn't have a plural, typically). It's like in English, you can say "I like water(in general)" or "I like the water(here in particular)", "Ich mag das Wasser" is fine. If you don't want to use an article with a countable noun like "apple", you would have to say "Ich mag Äpfel" (I like apples).
If you translate these two english: I like the apple. and I like water, and read them out loud, I think you'll answer your own question. It sounds odd to say: "I like apple" or "I like the water." (to me it does anyway)
If you already speak English as your first language, then the German articles should feel pretty natural. As others have said above, the article is dropped before mass nouns just like in English (mass nouns are things like water, flour, and rice that can't really be counted out).