How come this is card drawing? Isn't drawing cards more of ? When I think of pisces, I think of fish swimming away.
It was originally to increase the hand size and drawing (to promote the flowing flexibility of water), but there was not space for both and increasing hand size alone is too situational to be balanced.
The reliable drawing of cards is in Time's domain, but the situational drawing of cards isn't. See how Purify is Water's only form of healing (as opposed to healing in Life and Light) and how Ice Bolt is water's only form of direct damage (as opposed to damage spells in Fire and Air).
The intended theme of this mechanic is high-tide/low-tide in that you keep all your cards stored in your hand or you play all of your cards, optimally using as little time as possible for switching in-between.
Also, your idea of using the horoscopes for elements is brilliant. I just may steal that idea.
You go do that.