The Tail of Emily Windsnap by Liz Kessler

Emily Windsnap lives on a boat, but her mum has always been oddly anxious to keep Emily out of the water. 
 
When she's finally allowed to take a swimming lesson, Emily makes a startling discovery - about her own identity, the mysterious father she's never met, and the trilling possibilities and perils shimmering deep below the water's surface. 
 
Life wil never be the same again for Emily Windsnap...