A good rule of thumb: if its busy at Disney and Universal, it will also be busy at SeaWorld.
That said, 'busy' can be managed much more easily at SeaWorld than at the Disney parks. The shows accommodate large numbers at one time. Quick Queue can be purchased for the rides, if needed, generally at a fairly reasonable price (and is free for SeaWorld official hotel guests). Reserved show seating is also an optional add-on, and can be quite worth the cost in peak periods.
You will also have the Christmas Celebration events going on, which will be well attended are are WELL worth taking in. SeaWorld has one of the best, if not THE best, Christmas line-up for visitors in all of Orlando (and we've done Christmas in Central Florida 3 times, so we've experienced almost everything from SeaWorld's Christmas Celebration to Disney's Mickey's Very Merry Christmas Party to Universal's Grinchmas and ICE! at Gaylord Palms). SeaWorld's holiday offerings are free with a regular park ticket, so plan to go if at all possible. Their Oh Wondrous Night show is NOT to be missed, in particular. The whole park is just absolutely incredible during the holiday season!
