we took the BUS SYSTEM right from resort last year. You go to transport HUB at MK and jump on LYNX bus and they take you straight to Sea World.
It is CHEAP (like 2 bucks each way), disocunt for kids if I recall correctly...it is easy to do, it is basically door to door, but I would not do it with real little ones as you walk from parking lot to entrance (4-5 minute walk) with strollers and such, I think it may be a pain, maybe not though.
Also, it is PUBLIC transport, NOT disney bus so the atmosphere is NOT "disneyesque", these are regular people going to work.
We just found it to be easier than checking out to check in somewhere else and checking out again.....it was great. By the way it was a fun day at Sea World!! If you google LYNX and then call them they are VERY helpful with bus numbers etc....it was actually simple........