Mary Pope Osborne - Dolphins at Daybreak

The ninth book in the Magic Tree House series (1997)
A novel by Mary Pope Osborne
Morgan le Fay will make Jack and Annie masters of the tree house if only they can solve four riddles -- which will take four books, of course! Dolphins at Daybreak begins the third set of four books in this magical (and increasingly popular) series! Jack and Annie are off in the Magic Tree House again, this time to a whole new world under the ocean. Complete with a giant octopus, a hungry shark, and dolphins to the rescue, this Magic Tree House book delivers an underwater adventure kids can dream about.