Sea World generally opens at 9 am on the nose. You can get in at aroun 8:30, but you can only stay around the shops and not the main park area. Discovery Cove opens at 9 am and crowds are rarely an issue since they let in 1,000 guests a day.
If you want to ride the rides at Sea World, go straight to them as soon as the park opens. Rides like JTA and Wild Arctic can get long lines as the day progresses.