Yes, you can do Sea World in 6-7 hours. I went there by myself on a business trip in May as a "scouting trip" to prepare for a trip my family-of-four will take in January.
I arrived at 1pm and left at 6pm. Got to see the Shamu show and the Dolphin show. Walked around the whole park. Granted, I did NOT ride the Wild Arctic, the roller coaster, or their Splash-Mountain equivalent ride (sorry, forgot the name).
I had a great time, though, in the time I did spend there. I'm sure, of course, you could spend a whole day or two there to really experience things. However, if you're on limited time but are still interested in spending the money to go, it would be worth going for 6 hours.