Since DTD does not open until 9:00 am, they do start around 8:30 am or so, with the exception of the resorts closest to DTD. We stay at POR and I have never been able to catch one before 8:45 am.
Yes, if you want to get to SeaWorld earlier than about 9:45, you'll have to catch the bus at TTC, or take a taxi to DTD.
Don't forget you'll need exact fare of $2 a person each way, unless you buy bus tickets online ahead of time at golynx.com. (Don't order the "AdavantAge" passes; they are for seniors & handicapped with special id.)