I'd suggest either a taxi or find out transportation ( busses) that would take you to SeaWorld and use their "curtesy" transport. With DC , IMHO, you DO NOT WANT TO BE LATE, so I would take the taxi option in the morning and see how you feel at the end of the day. Although for the cost of the taxi there and back, you could hire a car for 3 or 4 days and use the extra days to visit SW and go shopping.