We do sometimes use the article when talking about water. Not all the time, but it does happen. When you see the article in the German sentence, you should include it in the English translation. Keep in mind that some Duolingo sentences also drop the article, so when you see those, you can then omit the article.
I thought about this too. It sounds incredibly strange, but I was thinking that as the sentences become more complex, we would need the "the". For example, the child is drinking the water she gave him. It would be necessary there. I also believe English has a lot of slang that other languages don't. This is more precise language.
Why it is Das Wasser, and not "Den Wasser". Emphasis is on water so can it be den Wasser?
Ok so water and milk won't be affected. But apple will be considered as it is masculine. Right?