It is definitely an answer to a previous question in Polish. It is correct to place an answer to a question and explanation in one sentence if divided by comma.
There should be two separate sentences to convey the same sense in English.
Napijesz się kawy? Nie, (dziękuję, bo) piję wodę -
Would you like some coffee? No (thank you). I'm drinking water.
Yes, I am drinking water (correct sentence) - Tak, piję wodę
No, I am drinking water (incorrect sentence) - zdanie nie ma sensu
No, I am not drinking water (correct sentence) - Nie, nie piję wody
Well, this is the beginning of the course, there is very little vocabulary and grammar introduced so far - many sentences will not feel that natural. This looks like an answer to a question like "Do you drink beer?" - although I guess it would feel better in Present Continuous. I changed it so that will be the default version.
As for "I will drink", that would be "wypiję", but it implies drinking (totally) something specific, like a glass of water. "Będę pić wodę" (I will be drinking water) feels more natural.