"ваша" is the more polite possessive pronoun and should be used in conversation with people you don't know and people older than you. You can say "Это твоя вода?" или "Это вода твоя?" as well. The possessive pronoun твоя/твой often used in conversation with friends or parents. The same rules can be applied to Ты - thou (obsolete/impolite/informal) and Вы (singular) - you (polite/formal/singular).