Mary Pope Osborne 
Magic Tree House 9: Diving with Dolphins [EPUB ebook] 

After a break from their trips in the tree house, Jack and Annie are delighted to find that it’s back! Morgan le Fay needs their help, but they’ll have to become Master Librarians first, and that means solving four riddles. For the first one, Morgan sends them deep into the middle of the ocean. Jack and Annie ride in a mini-sub and meet jellyfish, sea horses and an octopus. But there are some scary sharks in the deep and the friendly dolphins have to come to the rescue. . .