"Bois-tu l'eau ?"
You don't need to hear it because the subject (tu) is placed after the verb (bois), which is the interrogative form.
No because "do you dink some water?" is "bois-tu DE l'eau ?" (a certain quantity of water, not this specific water).
The first one is a question and the second is an affirmative, if I'm not mistaken.
click on the link, then on the small loudspeaker. http://translate.google.fr/?hl=fr&tab=wT#fr/en/bois-tu%2C%20boit-il
You are right, it is difficult to hear the difference between "bois-tu l'eau" and "boit-il l'eau". However, in real life, chances are that the question would be said this way : "est-ce que tu bois l'eau ?" or "est-ce qu'il boit l'eau", which may be a bit clearer in oral.
When the form of an interrogative sentence is the simple one, ie <verb first + pronoun >, you always use the hyphen sign.
You may even have to use 2 of them, every time the verb ends with a vowel, to ease the liaison:
mange-t-il ? pense-t-elle ? danse-t-on ? va-t-il ?
why is it this could not be "bois-tu l'eau" OR "boit-tu l'eau"? without a reference how do you tell?