EBh, I believe it's because 'usted quiere' = do you want. If you placed usted at the end of the sentence, it would be 'want to drink water you' instead of 'you want to drink water'. It becomes a question when ?? are placed indicating that your statement is indeed a question.
Statement: Usted quiere beber agua. = you want to drink water.
Question: ¿Usted quiere beber agua? = do you want to drink water?
I am no expert, so if a native speaker or moderator corrects me, please listen to them over me :-)
Thanks for the note. I reported another similar issue (didn't accept "Señora, ¿bebe vino?" as a translation of "Ma'am, do you drink wine?"), and I'll report any other such situations I come across.
Edit (May 16, 2019): Duolingo emailed me saying that "Ma'am, do you drink wine?" can now be translated as "Señora, ¿bebe vino?". Not sure about “Sir, do you want to drink water?” though.
Jane, they are different forms of the same verb. Remember that you have to conjugate verbs in Spanish, i.e. the verbs change their form depending on who carries out the action.
Beber is the infinitive (unconjugated) form, which you usually use if you already have a conjugated verb in your sentence, like quiere here. Bebe is conjugated for él/ella or usted, so it means "he/she is drinking" or "you (frml) are drinking". Bebes is the tú form, i.e. "you are drinking".