Creep from the Deep (Goosebumps Horrorland #2)

Billy and his sister, Sheena, unwittingly dive into a terrifying mystery when they accept a surprise invitation to HorrorLand--the amusement park that always lives up to its name. But first Billy and Sheena must sink or swim aboard a doomed ship that happens to be underwater--with a captain who happens to be dead. At least, he used to be!