I recently read a book called "The Mutation." It is the 36th book in the Animorph series by K.A. Applegate.
In it the Animorphs(a group of kids with the power to turn into any animal they can touch) find out that their arch enemy, Visser 3, is trying to make a huge ship/submarine to reach the bottom of the ocean. Visser 3 is trying to reach a ship with technology so advanced that with it he would be able to destroy Earth. The Animorphs all turn into into huge Orca whales to stop him. They all just keep on ramming into the ship to try to destroy it. After they think they destroyed it they see some creatures taking the ship away. They follow the ship because they want to make sure it is destroyed. They find a huge cavern filled with old ships the underwater civilization have collected and preserved. Eventually they get captured while one of the Animorphs gets separated from them. One of their friends saves them and they try to get on the ship to escape and after that to destroy it. They don't know if Visser 3 is still on the ship but take that chance.
Read this book to find out what happens next!
This book is fantasy because there is no such thing as people who can turn into animals.