Seal Song by Andrea Spalding, illustrated by Pascal Milelli (Orca Books, 2011).
I love the illustrations in this book. It's another selkie story (like Selkie Girl by Susan Cooper which I reviewed here) but what makes this one unique is that it's about children. A boy named Finn meets the strange and mysterious Sheila, and soon the fishermen in town are saying that she's a selkie. When Finn goes out to sea and gets caught in a storm, Sheila goes into the sea to rescue him, thus having to give up her human form and stay a seal.