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).