I love Christmas Day at Sea World. This is always my number 1 choice for Christmas Day in Orlando. Although busy, it's not nearly as busy as the Disney parks on this day. Crowds on Christmas Day here too tend to be a lot lighter than crowds between Christmas and New Year. It's very festive -- wonderful music, shows, and theming -- even some snow generated at the end of the day. And this is such a great park for a multigenerational group, if your spending Christmas with extended family down there.
I personally have found Christmas Day at SeaWorld less busy than summer Saturdays, October Saturdays, and days during spring break if this helps you any for gaging crowds.