1) If you take I4, Seaworld is about 10 minutes from Disney depending on Traffic.
2) I always say you should do 2 days at Seaworld so you have an extra day to go back and re see anything you really enjoyed or may have missed. If you are visiting Labor day weekend, then You will definately need 2 days at Seaworld to be able to see it all. Labor day weekend is a busier time of year so you will get to see Seaworld's "Summer Nights". Summer Nights at Seaworld includes Nighttime Shamu Shows, Sealion Shows which are different than the day shows. It also includes a Fireworks show at the end of the night and there are various activities scattered around the park, for example, a Live DJ and a dance floor. Any other day in September except Labor day weekend, you will not get these special shows. If you are going any other day in September except Labor day weekend, it is so slow that you could get away with doing one day. But like I said, I always prefer 2 just incase. If you are going Labor day weekend and you decide to go two days, then split the days up. Do a park opening (9:00am) until about 4 or 5pm on the first day to see all the daytime shows and exhibits. And then the 2nd day do something like 3pm-Park close (Probably 10 or 11pm) so that way you can experience all the nighttime shows and attractions.
3) Some "Must Do's" include:
1. Dolphin Feeding.... Do the first feeding of the day (9:30am) It's the least crowded and the best feeding time because it's the dolphins breakfast.
2. Shamu Show "One Ocean" .... If its Labor day weekend, the Shamu Night show is "Shamu Rocks" which in my opinion is better than One Ocean.
3. Dolphin Show "Blue Horizons"
4. Dog and Cat show "Pets Ahoy"
5. Sealion Show "Clyde and Seamore Take Pirate Island" .... If it's Labor day weekend, the Sealion night show is called "Sealions Tonight". Sealions tonight is a funny spoof on the Seaworld day shows, so if you see this show, make sure you have seen the majority of the Seaworld day shows or this show won't make sense.
6.Turtle Trek .... A brand new Manatee and Turtle exhibit which features a 3D experience. This exhibit Opens this April.
7. Journey to Atlantis .... A water ride like Splash Mountain
8. Kraken .... A floorless Rollercoaster
9. Manta .... A flying coaster which in my opinion is the best coaster in Florida.
10. Sealion Feeding
Any other questions, feel free to ask! Have a great trip!