Gordon R Dickson 
Secret Under the Sea [EPUB ebook] 
Under the Sea book 1

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 first Robby Hoenig adventure, Secret Under the Sea, an aquatic visitor from another world is threatened by a criminal gang…Gordon R. Dickson’s Robby Hoenig adventures are timeless young adult stories in the tradition of Arthur C. Clarke’s Dolphin Island.