Remember Sea world only gets a portion of the crowds WDW gets.
Seaworld even on a busy day, will seem like a small park compared to the zaniness packed to the gills Main Street in Magic Kingdom.
Think of it this way, you will be coming from what some say the #1 tourist attraction in Central Florida, to what maybe #4 or #5 (just a wild guess)
Thanksgiving week you will have people coming for the week, since most schools have off portion of that week or all of it (like my kids)
You will have some crowds, I'm sure you can find some attendance data somewhere, and find out what the usual attendance data for Seaworld vs. WDW (namely Magic Kingdom) and go from there. Some Central Florida guidebooks print that information.
Will it be horrendously busy? Don't know. It may not look busy compared to the business of Magic Kingdom on Thanksgiving week. (been there, done that, don't want to do that again)
But if you follow the simple plan of getting there at park opening, wait for the rope drop, get in line for the rides, and see an early show, you will have half the battle done before the crowds come at 10:30 / 11 am.