Learn Hardcore Portuguese (Brazil): Eu gosto de beber água.

In Portuguese, the verb gostar is typically followed by the preposition de before another verb in the infinitive or a noun. Think of gostar de as a single unit meaning to like (something), so you need the de there.

Can I say Eu gosto de beber uma água instead of Eu gosto de beber água?

Yes, but it changes the nuance. Eu gosto de beber água is a general statement about enjoying water. Eu gosto de beber uma água can sound more like you want “a [bottle/glass of] water” in a particular moment. However, in most cases when talking about water in general, you don’t use the article.

Is there a difference if I drop the Eu and just say Gosto de beber água?

No big difference. In Portuguese, the subject (eu, você, ele, etc.) is often dropped because the verb conjugation itself indicates the subject. Eu gosto de beber água and Gosto de beber água have the same meaning; using Eu can add a bit more emphasis on yourself.

Why is there an accent on the á in água?

In Portuguese, água has the accent to indicate which syllable should be stressed (“Á-gua”). Without the accent, the pronunciation could change. Accents can also help distinguish between different words with the same letters but different meanings or stress patterns.