E. Nesbit - Wet Magic (version 2)

Are mermaids real? Well of course they are! Just ask the four very English children who, when on vacation by the sea, find out about one being held in a circus and, since 'they die in captivity' decide on a daring midnight rescue. Is she a real mermaid? Well, help read this book and find out. Oh, and they fight sea monsters, engage in underwater battle and lots of other exciting and magical stuff. They are all mostly brave and helpful. If you like magic and water and peril and heroism, and the writing of E. Nesbit, this is the book for you.