I think it will be before my husband and I started having kids we went to parks like that on holidays. They always have a normal amount if not more of people. A lot of the locals I talked to at them said it was a tradition to go to them.
I took my son to the zoo on Tuesday and it was jammed packed with kids (I'm assuming since they are out of school on break. My son is two so im not sure if and when they go on break lol). There was alot of people traveling for the holidays too. I would think that Sea World would be busy too, but if you go when they first open you can knock out quite a few things before it gets too crowded.