Sea World is very manageable; because of the shows, you get to spend some good chunks of time sitting down and resting It is not a huge place, so it's not exhausting to walk around; however, if you have young kids, a stroller is a good idea. There is a large play area for kids that includes water play, plus the splash zones at a few of the shows, so if you think you'll want to do any of those, make sure you have dry clothes along, or at least a towel. In the summertime, it can get pretty hot; hat and sunscreen are essential.
Lots of people go very early to get seats for the shows, but pretty much every seat is a decent one, so don't feel you have to rush.