The Sinking — K.M. Robinson

When a quirky older woman pawns a fancy seashell necklace at her mother's antique shop on the pier, Cara doesn't think much about the story the woman spins about the wearer turning into a mermaid.

On her way home, she accidentally drops the necklace into the ocean and is swept out to sea where she meets Quay--a merman who volunteers to take her to his mother, the sea queen, to help her get her legs back.

Cara soon learns that it's Quay's eighteen birthday--a day that has been a curse for his family--and is meant to be one for her too. Now she must fight to survive the sea with Quay at her side.