Franklin W. Dixon 
Treacherous Tide 

Brother detectives Frank and Joe dive deep into the Florida Keys to find a missing person in the twenty-first book in the thrilling Hardy Boys Adventures series.Frank and Joe Hardy have been invited to take part in a conservation mission by a marine biologist at the Bayport Aquarium. The boys get to go on an all-expenses paid trip to the lush and beautiful Florida Keys to help save sharks! But the trip’s purpose is suddenly sidelined when a paddle boarder goes missing and rumors start that the disappearance is the result of a shark attack. After spending so much time learning about the local shark population, the Hardy Boys aren’t buying the story. It will take all of Frank and Joe’s recently acquired knowledge about sharks and their habitat to solve this case. Will they be able to discover what’s really going on before it’s too late?