Goosebumps Book 19 Deep Trouble

Billy and his sister, Sheena are visiting their uncle Dr. Deep on a tiny Caribbean Island. It's the perfect place to go exploring underwater and Billy's ready for an adventure. There's one rule to remember, 'stay away from the reefs', but the reefs are so beautiful, so peaceful that Billy can't resist but he's not alone in the water. Something that's half-human-half fish.