The Magic In The Half Torn Map by Alisa Bargeski
At the end of the 9th grade, Katie Terosie learns that her family is moving to St. Thomas in the US Virgin Islands. She is devastated about the move and leaving her friends in Minnesota. That is, until she meets her new neighbors, Seth and his sister, Lauren. While snorkeling, Katie finds a map in a bottle at the bottom of the Caribbean Sea. Katie, her younger sister Anna, Seth, and Lauren decide to follow the map. They discover that they can magically breathe underwater and, in a short time, find themselves in a tricky position. They are in a world of mermaids, mermen, and all kinds of dangers. The only way to get home is to find the King of this magical world and ask for his help. Will he choose to help or will they be forced to stay like all the other humans - as prisoners?
This story consists of adventure, friendship, and romance. Though they have good times and bad times in the underwater world, their only mission is to somehow find their way home.