I don't think so. Given the verb has to be in the second place, and the subject "het water" is already in the first place, "al" must be an adjective for the subject. If "al" is indeed used as an adverb "already", it should either come after the verb as in the simplest form "Het water is al van u" or be placed in the beginning for emphasizing purpose as in "Al is het water van u".
Thanks for the confirmation :)
And yesz you're right: when I was just beginning with Dutch, my partner (native speaker) insisted that it was de boek :') you can imagine my confusion! It turns out that speaking primarily English for more than a decade has affected my partner's Dutch a bit.