Keeper

Keeper is a breathtaking, magical novel from National Book Award finalist and Newbery Honoree Kathi Appelt.
To ten-year-old Keeper the moon is her chance to fix all that has gone wrong ... and so much has gone wrong.
But she knows who can make things right again: Maggie Marie, her mermaid mother, who swam away when Keeper was just three. A blue moon calls the mermaids to gather at the sandbar, and that's exactly where Keeper is headed - in a small boat. In the middle of the night, with only her dog, BD (Best Dog), and seagull named Captain. When the riptide pulls at the boat, tugging her away from the shore and deep into the rough waters of the Gulf of mexico, panic sets in and the fairy tales that lured her out there go tumbling into the waves. Maybe the blue moon won't sparkle with mermaids and maybe - Oh, no ... "Maybe" is just too difficult to bear.