Gordon R Dickson 
Secret Under the Caribbean [EPUB ebook] 
Under the Sea book 3

Dive into adventure in the last unknown territory on Earth – the sea. Join Robby Hoenig and a cast of characters as unusual as any ever met on a distant planet . . .In the third Robby Hoenig adventure, Secret Under the Caribbean, the key to a baffling puzzle – and the truth about a mysterious monster – are locked inside a sunken Spanish ship . . .Gordon R. Dickson’s Robby Hoenig adventures are timeless young adult stories in the tradition of Arthur C. Clarke’s Dolphin Island.