Since you need to transport an ECV, you will need to use a separate shuttle (a smaller van). It is also free, but you have to let the concierge know you will be going to Sea World 24 hours in advance. They will arrange for the accessible van to come get you at the same time the bus picks up everyone else.
We used this shuttle when we were in Orlando (at the Hard Rock) in March. My husband and I were the only ones in the van. We let the driver know when we wanted to be picked up at Sea World and he was there right on time and took us back to the HRH.
Just let the concierge know when you are planning on going to Sea World.
Have fun!
Jennifer