If you book a rental through
Alamo/National, you can get your car at the Car Care Center, or from the Dolphin..you do have to make that choice when booking though. The CCC will send a shuttle for you. AGain, you have to let them know you want a shuttle to and from their office.
At the Dolphin, you can just head to either DHS or Epcot and go to the Dolphin and get your car.
There are various car rental companies in various DTD resorts.
Budget, for instance, is at the Doubletree..and they also have a shuttle.
No rental company brings the car to you though.
There is a Lynx bus that runs from the TTC and stops at SeaWorld's parking lot entrance. It's about $1.50 pp, each way....need exact change. I think they leave at 20 past the hour and 10 before the hour, but that may have changed. You do have to let the driver know, when boarding, that you want to get off at SeaWorld.
If going to US/IoA, you can book the Mears shuttle....$18 or so pp, r/t. It needs to be booked the day before you want to go..you can call from your resort room. They will give you a pick up time. You can either set up your return time then, or call them a few hours before you are ready to leave the parks.
A cab will run you about $35-40 each way. Car services will be about the same.