If you did not want to rent a car, Mears has a shuttle that goes to Sea World or Universal. They will pick you up at your resort, the last time I looked into the cost I think it was $28.00 per person round trip.
You can also rent a car at the car care center in WDW, the Swan or Dolphin has car rental as well as the Holiday Inn in DTD. You can just rent a car for a few days, it may save money in the long run.