I think he or she would definitely do great at Sea World. My brother just took his two boys (6 and 3) and they had a great time.
Never been to Aquatica, so I can't comment on that.
The only thing about Discovery Cove is that it is very expensive and I don't believe they're old enough to do the whole dolphin experience thing. If you can't do that, IMO it's not worth the admission price. Sure you can swim and see the stingrays and see other fish, but you can do that other places for a lot cheaper.
So basically I would skip Discovery Cove until they're older. JMO.