"You drink your water."
Translation:आप अपना पानी पीती हैं।
Aap is gender neutral, but you know the gender of the person you are speaking to, so need to use the correct gender. And if you were addressing them as Aap, and it was a commend (which in this case it isn't, it's merely a statement) wouldn't you use the polite form pijiye, not piyo?
Tum is used when addressing colleagues or people younger than you. It is an informal way og addressing someone when saying you. Aap is when you address older people or someone you want to show respect to. A bit like saying Mr. So and so in American English. Unfortunately that concept of respect is completely lost in British English as you address even a 80 year old by name in UK. Hindi is a very respectful language and you will find adults addressing even little kids as aap.
You can also understand this fact by looking at language origin and common words. In most Indo-European languages (English, Spanish, and Hindi in this example) forms of you are similar. Thou (English), Tù (Spanish), and तु (Tu) (Hindi) are all non-formal and sound similar, while You, Usted, and आप(Aap) are formal and also sound similar.