Take a cab. Easiest if not the cheapest way there. You are probably looking at about $25 - $30 for the cab each way.
The advantage is no stress. When you are ready to go, Bell Service will have a taxi there in just a few mins. When you are ready to return, there are cabs waiting in a line to pick you up at SeaWorld.
No car to return, no public transportation. You will save hours of time and a hundred dollars worth of hassle.