I have been to Sea World on NYE, though we didn't stay until midnight.
Crowds throughout the day were managable, Possibly because the park is so big. Also SW is mainly shows, so long as you got there early enough, there was no problem. We actually did more on NYE than we had done on previous visits. Rode Kraken with no wait around10-11am. Saw all the shows, including a couple we hadn't seen before. The longest wait all day was around 25 minutes for Wild Arctic, we were standing in that line at 7pm, midnight at home, quite surreal thinking about the folks back home at that time.
There were more people coming in from around 6pm, just for the evening I guess, but there were also people leaving who had been there all day. We left around 8-8.30pm as we had children with us, and we had never intended to stay til midnight. I would definately do SW again on NYE.