Mears is your best bet for transportation to Sea World and/or Universal.
Detailed Instructions
The day before you want to go to Sea World, pick up any house phone at WDW and dial "0".
When the Disney operator answers, ask to be connected to Mears.
When the Mears person comes on, tell them you wish to go to Sea World the next day, how many in your party, which resort you are staying at, and approximately what time you wish to leave.
The Mears operator will give you a pick up time.
The cost is around $13 per person, round trip, cash only to the driver.
The day you are scheduled to go to Sea World, proceed to your resort front entrace. Some resorts have Mears pick up at the bus stops (mainly value), some have Mears pick up at the front entrance.
When you arrive at Sea World, the bus driver will give you a return trip ticket which has a phone # on it.
About two hours before you are ready to leave, call that phone # and schedule a pickup time. Use a pay phone or cell phone.
Return to the location where you were dropped off that morning at the time Mears gives you.
Hope this helps!
Carol