Are you staying on site at WDW? If so, you can jump on the Mears shuttle to Sea World - no charge. We did this on our last trip and it was very convenient.
Ish, no charge? Are you sure? I have read on the Seaworld bus that it is best to take the Lynx bus (1.25 each way). I will be going to Seaworld as well in December and am still looking for the best transportation.
I'm not sure about Lynx, but if you are staying on site at WDW you can take the Mears shuttle to Sea World. We did not have to pay anything. There was a pretty flexible schedule, too although I cannot remember the exact hours now.
We were staying at POR Riverside and the bus came right to the resort i.e. where we caught all the WDW transportation. We had several return times to choose from and the Mears shuttle just picked us up in the Sea World parking lot and brought us back to WDW. I think we arrived at Sea World around 9:00 AM and returned to WDW around 4:30 PM. The shuttle was not crowded at all and we were very satisfied with the service.