Hi @LilithKitty,
That’s a great question. For me, I view correspondences as a way to further enhance and direct spellwork. We don’t need anything other than our willpower to cast, but by incorporating certain influences, we can get help our spell manifest faster, stronger, and in more impactful ways. This can be done by mindfully including certain herbs, aligning casting time with moon phases or planetary movements, using certain crystals, etc.
So which herbs match up with which astrological influences? You’ve already got some great ideas from Amethyst and Water, if I can add a few more, I have a list of Zodiac teas made from herbs you may find helpful. A note that these come from a Slavic pagan/herbalist practice (and correspondences tend to vary from one tradition to another). Feel free to take a look and use any that resonate with you! 
Blessed be! 
