book your seats the night before (at least) at the hotel lobby desk.
the shuttle will stop at the 3 onsite hotels.
you need to guarantee your seats so don't wait until the last minute to do so.
you can not book the seats on the day you want to use it.
save money and take the shuttle to seaworld in the morning.
if you decide to leave earlier than when the return shuttle will be, take a cab back to the hotel.
it will be less than $20 for that.