We were there the first week of Dec. last year. The park was EMPTY! It was so great. In the kiddie-ride area, the ride operators were literally waiting for kids to walk up so they could start the rides again. There were several times my kids were the only ones on a ride!
The only time we noticed a crowd was at the Polar Express ride. We (and probably 90% of the people in the park) went there right after the Shamu show. DH even did the manta roller coaster in the middle of the afternoon in under 10 minutes.