They have something like that I think, I know I have seen Seaworld resort buses, though of courses the resorts do not belong to them. There is also a free shuttle that runs between Aquatica and Seaworld. It probbaly could be run better though. I take public transportation, and so often I am sitting at Universal Orlando and amazed at the frequency with which the buses to Cabana Bay come.
Anyway, I think what Seaworld should do is create a whole new parking structure, a multi-level structure, build it on the ground south of Aquatica (I am pretty sure the vacant land is their property), use this as the parking for all the parks, and allow the old parking space of aquatica and seaworld to be used for expansion of the parks. Build a pedestrian bridge across I-drive to go from the parking structure to a newly created entrance somewhere for seaworld. --- Of course, chances of this happening are close to zero probably lol