Creep from the Deep (Goosebumps HorrorLand #2) by R.L. Stine

It’s a whole new ride from master of horror and bestselling author R.L. Stine–with a story so fiendish that it can’t be contained to just one book!

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!