First, you probably will be able to answer at least the simpler questions your child has about water. But if you feel really out of your depth, try doing some research. You can tell her, “Let’s look it up together!” You can make it a fun research project that your child can take part in.
Try not just the Internet, but perhaps books about water or an encyclopedia. Make it a fun trip to the library. You can ask the librarian, too, giving your child an example of requesting help. Make it fun for your child and you’ll encourage your child to keep asking questions. This is a good thing and supports his sense of curiosity.