The Tail of Emily Windsnap, by Liz Kessler, 2003

For folks living on a houseboat, Emily and her mom are more than shy of water. So when Emily takes her first swimming lesson in gym class and finds her legs fusing into a scaly tail, no one's more surprised than she. Why didn't her mother tell her she had a merman for a dad? And why does that creepy Mr. Beeston keep coming over to feed her mother doughnuts? Light middle grade fantasy.