I think any of the theme parks are difficult to do in 1 day, especially if it is your first time there. It depends on what shows, rides and attractions you hope to see; the park hours that day; if your family are early risers and can beat the crowds when the park is less crowded in the morning; and, the weather. I am planning our 1st SeaWorld visit, and am thinking a 1 -1/2 days. We had our 1st family trip to Disney last year for 4-1/2 days - and although I was very well prepared - we still only accomplished about 1/2 to 3/4 of what I was hoping to accomplish in each of the parks. If you want to get back to Disney at night, I would suggest picking the most important things you want to do at Seaworld, have some flexibility in your schedule if run into long lines at any of the attractions you hope to see. Do not get frustrated if you can not get as much in as you hope. Hopefully, you will have another trip in the future for anything you miss.