We've taken the city bus from TTC (went POP to Epcot to TTC). The trip there was fine and was really reasonable. But then we couldn't find the bus on the way back and took a private shuttle that came to the bus stop (that was unplanned and a little weird, but worked out). That's the cheapest way.
We've also taken a taxi to and from our resort. That was more expensive but for us was much better time and headache wise.
If you have a car, just follow the signs to I4 and take it north, following the signs for Sea World. It takes probably 30 minutes (or less) to get there, starting out from your resort. This is the route I've taken a couple of times.
We took the public bus from DTD. The stop is right across from the WDW bus stops near the shopping area of DTD. It was very cheap and easy. There were no changes and the bus drivers were very helpful. We told them where we were going and they signaled when it was time to get off. Total for 2 adults $8.