Alas, I didn't like this one as much as
So You Want to Be Wizard, and, judging by the comments, most people seem to like this book best.
Nita and Kit are summer vacationing at the beach; they end up encountering a whale wizard (nifty!). S'ree (the whale wizard) lets them know that the Lone Power is looking to wreak havoc again, and that she needs their help to re-bind him beneath the ocean, as was done long ago. Nita soon discovers that the world of the ocean will demand more from her than expected and that even the Pale Slayer (Ed the shark whose name is actually much longer that Ed) has a place in the ecosystem. All while trying to slip out under her parents' noses.
This didn't end up working as well for me because I tend to dislike environmental themes/ecological themes in fiction, along with circle-of-life themes. Yes, yes, everything has a place and everything is balanced and humans muck that up, and I knew that already. The themes may have worked much better on me if I'd been younger and not read more books with these themes in them, or if there had been a deeper investigation of them. Also, I figured out how the book would end many chapters before the fact, which took out a great deal of the suspense and the emotional power of the ending.
Also, stories in which kids disobey their parents or must sneak around under their noses always make me uncomfortable, probably because I was the type of kid who was scared to death of being yelled at by my parents. This is definitely an idiosyncratic reaction, but as this is the book in which Nita must confront the intersection of her wizardry and her parents, I don't think there was anything Duane could have done about it. For what it's worth, Nita's parents don't act stupid and Nita and Kit don't act stupid, but I get uncomfortable just the same.
I suspect this is a really cool book for people who like underwater ecosystems and whales and dolphins, but since I like Manhattan more, I enjoyed the first book more.