Labor Day are generally crowded days, but I think the most crowded places in Sea World will be the rides -- Journey to Atlantis, Kraken, and the Wild Arctic simulator. The rest are shows and attractions. I would arrive about 10-15 minutes before showtime to get a good seat. In the case of Clyde and Seamore, I'd go 20 minutes before showtime to catch the mime. Everything else can be done at your leisure.
You can do Wild Arctic if you do the non-motion ride. It's the same movie as the simulator, but the seats don't move and the lines are usually shorter.