It's busy, but not what I'd call "Disneyland" busy. The good thing about the zoo and Sea World is that there are less attractions that actually require waiting in line. The zoo has the Skyfari and the bus tour (oh, and the pandas), and Sea World has its handful of rides, but other that that, you can move at your own pace. Some of the longest lines will be the restaurants! If you want a great seat for the Sea World shows, make sure you get to each theater in advance (don't show up at 11:55 for a 12:00 Shamu show and expect to have a good view in the summer). The other good thing about the zoo and Sea World is that they both have extended hours in the summer and are open at night with special shows and events.