The Tail of Emily Windsnap by Liz Kessler

When Emily Windsnap has her first swimming lesson, she finds out a big secret about herself. She's a mermaid. Part time, actually. She can turn into a mermaid when she goes into the water fully. When Emily makes friends with a full time mermaid, Shona, she finds out that her father who left her and her mother when Emily was an infant, didn't want to leave her and her mom. And, she tries convince her mom not to think about Mr. Beeston, as a "nice, charming" light house keeper, who works for the merman king who took Emily's father away. Read this book to find out where and why Emily's father is! I hope you love this book as much as I did!