The Sea Fairies
Trot and Cap’n Bill meet some mermaids who invite them to visit their palace underwater. Trot and Cap’n Bill accept the invitation and, after the mermaids give Trot the form of a mermaid and Cap’n Bill the form of a merman, they all go to the palace, where Trot and Cap’n Bill meet Queen Aquareine of the mermaids. But Zog the Magician captures the queen, Trot, Cap’n Bill, and Princess Clia, and it’s a battle of magic to see who will come out victorious.
As a warning, in my copy of this book a few little sections have the lines of text switched around, so you have to read it out of order so it makes sense. Zog is pretty sinister. The battles of magic in his castle are neat. Most of the book is more of Trot and Cap’n Bill visiting the ocean, but it picks up somewhat at the end with Zog.