moreligirl- As a native, water = eau and the water = l'eau, but Duolingo decided to accept both. Sometimes without context, it's not easy to give the correct answer. In English when you say, I drink water, it means some water, so I think that instead of accepting l'eau they should have accepted de l'eau, in reference of some.
I think you can't always directly translate English words and phrases for French ones. This is a classic case where in English we do not use the article always and so water I think assumes to be some water. Therefore I think it should be d'eau. My argument is that at the dinner table if someone says "water" they imply "Some water" more than "the water"
In English: Today the subject of discussion and thought is Water. (All water, the idea of water, potentially encompassing all the water that ever was or ever will be)
In French: L'eau. Le is used in it's general sense and is contracted to form L'eau.
Water when capitalized and in quotation marks it is likely to the the title of a book, program or subject of general discussion. In such circumstances le is the appropriate article.
However, in the case of Duo that is not a reliable guideline since they capitalize and place quotation marks on all their examples. I guess Duo's take is that the only way water would be introduced without any context is if it was used in the general sense.
De l'eau (I want some water) and l'eau (le used in the particular I want the water) require context. Of course, it's not helpful that Duo seldom provides context, so the absence context doesn't really give any assistance.