Aww
You might not need surging waves for your siren spellbooks though. Maybe you should try playing with her to see how you'd like to play her before you buy an expansion you don't really want for attack spells you don't particularly need in your siren spellbooks. She'll have plenty of options, I kinda doubt you'll really miss surging wave if you make a siren spellbook that doesn't need it.
Now that I think of it, I'd probably like to do something with surging wave on an elemental wand. Siren's call ability works on mages right? I could make them move towards me get rid of their armor if they put any on, put on some armor of my own, and just surging wave them over and over again until they die. Daze rolls help protect my Mage from being hit back and if they have a defense, being incapacitated opens them up for other more powerful attacks immediately after the surging wave.