Creep from the Deep - Goosebumps HorrorLand 2 (Unabridged)- R.L. Stine

Billy and Sheena always expect adventure when they join their uncle, Dr. Deep, aboard his hi-tech boat. What they don't expect is a treasure hunt leading to a 200-year-old sea captain ... who refuses to stay dead!
Just when they think the tide is turning, Billy and Sheena accidentally dive into a dangerous mystery. A chilling surprise awaits them in HorrorLand, the theme park where their worst nightmare is about to come to life.