Was there on Xmas day. Crowds not that bad. Got much worse as New Year's day approached. Might be same for tHanksgiving?
I find that SeaWorld absorb crowds better than many Orlando parks. So many shows that hold thousands of folks. If you are coaster folks you need to hit those at opening. Lines for dolphin feedings tap out on busiest days. Do Polar express while Shamu show is on.
I went Thanksgiving day last year - honestly I didn't think the crowds were that thick - it was holiday crowded but definitely manageable and enjoyable.