"L'eau est-elle bonne ?"
In this interrogative form, with the subject starting the sentence, it is necessary that you switch verb and pronoun. Otherwise, the other interrogative form would be "est-ce que l'eau est bonne", where the switch is made with "ce". And you cannot say "est l'eau bonne ?" as you would say "est-elle bonne".
Thanks for this answer, but I still dont see why "est l'eau bonne" is wrong? its because the water is something that you can not ask directly if is good?
the switch verb-subject only works with pronouns or "ce", not with a noun or a name, it does not matter if it is water or something else.
- est-il vert ?
- est-ce vert ?
- sont-elles vertes ?
- l'eau est-elle verte ?
- Marie est-elle là ?