I think Disney would sooner build a park in Shanghai before purchasing Sea World.
Some competition is required in the Orlando area. Don't forget, there are people that don't like Disney that much. If they come to Universal, Sea World etc, they might stop by at Disney for like a day. If Disney owns almost everything, there is also no real incentive to keep the quality up and prices down. I know Sea World doesn't really offer any serious competition to Disney, but it is still in the back of their minds.
Competition is healthy, and is an integral part in making sure we consumers to get ripped off.