For Sea World you want to follow the signs for 535, but DON'T TAKE IT. 535- Apopka/Vineland Road is a different road from International. The WDW exit you'll take is near Downtown Disney (where there's another McDonald's, which opens fairly early).
You'll see the road split off for I-4, and then you'll go through the intersection for 535. Just past that you'll see the signs for Int'l Drive. It's fairly easy, because if you go straight, the road becomes a toll highway. You'll see a lot of people heading left onto I-Drive.
About a mile up I-Drive you pass the Premium Outlets, Dixie Stampede Dinner Theater, a VERY NICE 24-hour Walgreens, and a Publix grocery store (oddly enough, not 24-hour).
All of 3 miles at most past that you'll see the turn (LEFT) for the Sea World entrance. (There's also a Ponderosa and a Denny's at that intersection.)
PLEASE NOTE: International Drive at that point looks very different from the North end of I-Drive. It's very quiet, and there's lots of houses and trees. Everytime I head that way I see so many tourists thinking that they're on the wrong road, and getting themselves lost.