The family and I were just at Seaworld/Busch Gardens, US/IOA two weeks ago. There is a bus that runs from Seaworld to Busch Gardens daily. I think it was $40/person. You might check on that one. We decided against it since we had a car. It was less than an hour a half drive.
Good luck! Have fun at Howl O Scream! We are going to the one in Williamsburg at the end of September!!