Shadow of the Shark (Magic Tree House: Merlin Missions #25) by Mary Pope Osborne

Jack and Annie are diving into danger when the magic tree house whisks them away to shark-infested waters in this NEW adventure in the New York Times bestselling Magic Tree House series!
 
It’s a dream vacation for Jack and Annie—or is it?
 
When Teddy, a young sorcerer, offers to send Jack and Annie on a dream vacation, they can’t wait to go. The brother-and-sister pair wish for a trip to a beach paradise, and the magic tree house whisks them off to the coast of Mexico. Everything starts out perfectly as they raft around a coral reef. But then a hungry shark attacks! And their dream vacation turns into a nightmare!