The Ticket and Transportation Center (TTC) is next to the Magic Kingdom parking lot and where you can board the monorail to there and Epcot.
So from Pop Century you have to take a bus to Epcot or MK, and then a monorail to the TTC.
#50 bus - Leaves the TTC at 18 & 48 after the hour. Returns from Sea World at 09 and 39 after the hour (until 9 pm)
At the TTC, the Lynx stop is located at the far end of the platform. Exiting the monorail or ferryboat, head out of the TTC towards Mk parking and bear left around the gift shop located at the corner. You will then walk straight to TTC bus parking. Stops are marked with the Lynx pawprint and are after the few Disney buses which still service the TTC. So the Lynx stop is to your left at the far end (but its actually very close). More than one Lynx route calls at the TTC stop, so be sure you get on the correct bus (the second will take you out U.S. 192 to Osceola Square Mall!!)
Fare is $1.25. They do accept dollar bills, however, the preferred method is five quarters. You can of course use nickels, and dimes as well. They don't take $5 bills or over.
Tell driver when you board at TTC that you want to go to SeaWorld.
Sea World is about the next stop after the TTC and the trip takes about 20 minutes. The bus route is basically I-4 from Disney (TTC) to the Sea World exit. There may be one or two Lynx bus markers after you exit I-4 but before you turn to pass in front of Sea World itself, but the correct stop will be pretty obvious. It's right next to the SW parking lot on the right, which you have to walk across to reach the entrance!
For the bus home - cross the two-way street at the crosswalk and pick up the bus on the OTHER SIDE of the street (on the other corner - Renassaince Hotel driveway is right next to the stop). There should be a sign with Lynx paw prints on it. You can check with outward driver where the bus picks up, if you want to be certain.
So it's a bit more of an adventure, but at that price......!! We did it in February with two small children and a stroller, and it didn't seem a big hassle - and I DO get hassled easily!! The timetable and price was correct for Feb 05, but it may, of course, change.
HTH