Also can't book that far out online. You have to call in. For us, we prefer to call in anyway. We use "Online" to do our research, but like to have an actual person on the line when we pull the proverbial "Trigger".
I will say that we have gone twice now in late November and we LOVE it. The weather is mostly nice, and except for a brief few days (Thursday - Saturday) around Thanksgiving, the crowds aren't too bad.
Last time in 2014 we went to Sea World on "Black Friday" and there was hardly a crowd at all with the exception of their new Penguin exhibit. We did want to see it but the line was over an hour long, so we decided against it this trip. Got to ride all of their coasters with minimal wait (5 - 10 Minutes, tops).