Salt & Venom (Blood, Bloom, & Water, Book Two) by Amy McNulty
An emboldened merman. The daring girl who agreed to be his champion. The upcoming battle that threatens to change everything they believe in.
Ivy Sheppard never wanted the ability to turn into a mermaid—though her wicked ice powers are nothing to sneeze at. Now her step-sister is her enemy, the fairy she once trusted may not be the ally she hoped for, and her new boyfriend wants her to transfer schools to avoid the vampires taking over Union High. Between dodging parents’ concerns, pretending she can stand to even look at the step-sister who sided with the bloodsuckers, and still aiming to end it all without violence, Ivy is in over her head. Unfortunately, these ancient enemies are itching to get the conflict started and Ivy may have no choice but to become the warrior the merfolk need her to be.
The second book in the Blood, Bloom, & Water series starts the war between fangs and fins in earnest as the merfolk plan their assault to bring down the unnatural undead vampires once and for all.