Fins are Forever (Fins, 2) by Tera Lynn Childs, 2011. Published by Harper Collins.
On Lily Sanderson’s eighteenth birthday she’ll become just a girl—still a mergirl, true, but signing the renunciation will ink Princess Waterlily of Thalassinia out of existence. That leaves plain old Lily living on land, dating the boy she loves, and trying to master this being-human thing once and for all.Why: I absolutely adored Forgive My Fins and need to know what is going to happen next...and what sort of trouble Lily is going to get herself into.
Now that Lily and Quince are together, mer bond or not, she’s almost content to give up her place in the royal succession of Thalassinia. But just when she thinks she has everything figured out, the waves start to get rough. Lily’s father sends a certain whirlpool-stirring cousin to stay with her on land. What did Doe do to get herself exiled from Thalassinia and stuck in terraped form, when everyone knows how much she hates humans? And why why why is she batting her eyelashes at Lily’s former crush, Brody?