The book I just finished reading is called "High Tide in Hawaii" written by Mary Pope Osborn. It is the 28th book in the Magic Tree House series.The main characters are Jack, Annie and Morgan. Jack likes to take notes and he is more careful than Annie. Annie likes animals and she likes going places and she is not as careful as Jack. She can always tell when the Magic Tree House shows up. Morgan is a magical librarian from Camelot.
Jack and Annie go to Hawaii in the Magic Tree House. Morgan left them a riddle in the Magic Tree House and they figured out that they had to build some sort of ship that never sinks and always floats. Jack felt an earthquake and then read in his book that when a earthquake happens there is always a tidal wave. Even though Jack had a hard time surfing the first time he tried, he decided to surf to Annie, Kama and Boka, some kids they met in Hawaii to warn them. This time Jack did better, because Kama told him before that it's better to stay on your stomach when you surf. The answer to Morgan's riddle was "friendship."
And if you want to see how it turned out you need to read the book.