You probably know by now, but its because they are talking about the general concept of water. So, (unlike English) in French you add the definite article 'la' or 'le' (translated to 'the') before the noun to show that it is the general concept.
E.g. "j'aime >les< chats" = "I love cats" ">les< pommes sont rouges" = "apples are red"
Only add "le/la" by itself if you can say "in general". E.g "in general, apples are red" works. But "I eat some meat" in general- doesn't, that would be translated to "je mange >de< la viande"
"I eat cake" can not be translated to "je mange >le< gateâu" because you can't eat the general concept of cake. If it is an undefined amount you say "je mange >du< gateâu" for "I eat some (an undefined amount of) cake".
Radulian, I was confused about this because earlier in the lesson I think it was explained that an unmeasurable amount needed to have "de l'" for "eau" and it meant "some water" even though we don't normally say the "some" in English. But in this sentance "Some water is a drink (or 'beverage')" doesn't fit. So I put "the water" and it was accepted by dl. But in English, we normally would say just "water" in this case. So I guess it's one of those things you have to learn by using French over a period of time.
There is a liaison between est and une. Est’s last letters should be silent and pronounced as “eh”, but the liaison condition is available so it should be pronounced « est [-t-] une ».