Well, my 16 month-old loved SeaWorld! She loved all of the shows that we took her to (whale and dolphin show, Sesame Street/Elmo show, Shamu, and some show that was like a Cirque de Soleil-type acrobatic show).
Also, I am glad that I brought my hands-free carrier for her because there are a lot of walk-thru exhibits there (sharks, polar bears, etc.) and they were too crowded for her to walk holding my hand. If you do bring your toddler, it is a good idea to bring a carrier or sling of some sort unless you are OK with holding them in your arms as you walk through exhibits for long-ish periods of time. You can't bring your stroller through these exhibits.