While it is possible to cover most of SeaWorld in one day, the show times make it almost impossible to take in all the shows. It does also mean you have to go at a pretty fair pace and not allow the kids time to enjoy exibits like the tide pool, feeding the stingrays,seal, dolphins or the excellent kiddies climbing/play areas. It depends on if you like to "commando" a park or if you like to take your time to "smell the roses". A lot of the exibits , like penguins, seals and dolphins you can easily spend 30-40 minutes at enjoying their antics. I've seen people at those exibits drag their kids up the stairs and say "look there are the seals, ok we've seen them , let's go" . I'm not saying they're wrong to view the park in that way, if it suits them that's fine, but I do feel they are not making the most of the park and they are also missing a chance to educate their kids at a time when any little bits of information they pass on is likely to stick because the kids can see the animals and they are more interested than sitting at home or in a classroom.
I would take 2 days for cover it all and take a reasonably gentle pace, particularly as if you're planning to buy a single entry ticket they usually are on a 2nd day free program. Don't miss the Pets Ahoy show, for adults it may be a little too sweeeeet, but kids of that age seem to love the show.