we were there last week for our first time ever to SeaWorld (and Universal)
If you can, purchase your tix via the SeaWorld website more than 7 days in advance of your visit. They offer a discount if you can purchase more than 7 days in advance. If for some reason you do not use them, they are refundable.
Then, just purchase for one day. When you leave, right at the gate, they offer a return for a second day free. We obtained the 2nd day free, but ended up not going back.
For us, it was darn HOT. We got to the park just as it opened, picked up a map and headed for all the shows. We were able to catch the top four shows and an air-conditioned lunch at Shark's, feed the Rays and Dolphins, and visit the Clydesdales and leave by 3:00. (a storm was just about to start as we were heading to the car, so we did good to head out about then.) We did not ride any of the three rides they have. We would have done those had we returned the second day.
HTH and have a great time!