Translation:You drink tea.
To my knowledge, 在 is used for action verbs. 我吃鱼 is perfectly fine (perfectly correct, perfectly natural Chinese) for meaning either "I eat fish" or "I am eating fish: 在 is not necessary, but may be added in order to emphasize or to specify 我在吃鱼 , "I am presently eating fish," as 吃 , "eat," is an action. Mutatis mutandis for 我喝茶 : it means either "I drink tea" or "I am drinking tea," but you could say 我在喝茶 to specify the latter if you feel compelled to do so.
I see French is one of the languages you are studying; 我吃 is like "je mange:" it means both "I eat" and "I am eating" (and it is not necessary to say "je suis en train de manger" when the simple "je mange" will do - although there are times when such a construction is indeed better).
我吃鱼 means "I eat fish", this kind of expression is rarely used, it appears in situations where somebody ordered takeaways, asked you which you prefer, chicken or fish? Then you'd reply 我吃鱼（的） in the context of "I want (the one with the) fish". Otherwise, you'd always say 我在吃鱼 as "I am eating fish".
Yes, you are both right: 在 can be used in exactly that way. Yes, 在 can indicate present action, but, it is not always necessary to do so. This sentence, 你喝茶 , even without 在 , already means either "you drink tea" or "you are drinking tea." In English, the distinction between "simple present" (aka "habitual present") and "continuous present" is so strong that the syntax changes to indicate one or the other; in Chinese, the distinction is not as strong: the same syntax indicates either, but 在 can be added to emphasize presently happening action if such emphasis is called for. I notice Holsen4, who asked the original question, also studies French; so, I will add, for comparison, that French does not distinguish present habitual from present continuous at all: "tu bois" means either "you drink" or "you are drinking," as does 你喝 , except, in Chinese, it is easier to specify "drinking rather than drink" by simply adding 在 ; whereas, in French, the specification becomes awkwardly wordy, "tu est en train de ..." or something along those lines, to the point that the French generally don't fool with the distinction at all.
Even if you prefer to use 在 this way, to emphasize present action, leaving 在 out of the sentence does not automatically make the sentence simple present instead: without 在 , the sentence can still mean either. So, what are you going to do? always use 在 for "-ing" and say something like 你喝茶可是你不现在喝茶 every time you want to specify present simple (or present habitual)? I certainly would not recommend that: generally, it's unnecessary, awkward, and a waste of time and effort. Just say 你喝茶 and relax, knowing that it means both or either; so, it's usually obvious "which“ it means, even if it matters at all.
In sum, 你喝茶 already means either "you drink tea" or "you are drinking tea" as is, without any changes. Adding 在 , 你在喝茶 , does mean, specifically, "you are drinking tea" rather than simply "you drink tea," but in Chinese, the emphasis is generally not required, more like in French, which almost never requires the distinction, and unlike English, which not only requires the distinction, but makes the distinction as a matter of course.
English speakers are so used to the automatic distinction that they can "get hung up on" whether a French or Chinese sentence is habitual or continuous: some English speakers, especially when first studying such other languages, feel they "need to know which" form of the present it is, when, in languages such as French and Chinese, the two possibilities are sort of "rolled into one:" the distinction is simply not as strong, or not made at all. French speakers studying English sometimes suffer the opposite problem: they don't understand why sometimes it's "you drink" and other times it's "you are drinking," and how English speakers just automatically seem to know which is correct. My advice would be not to use 在 in this way unless you really have to: instead, get comfortable with using verbs that can mean either.