I don't know the numbers on theme parks either, but its hard to imagine that these seasonal parks would get an extra million people a year to get to where SWO's numbers are today if they opened up the rest of the year. If you are looking at per day numbers, you'd have to look at the same time period, the peak season SeaWorld vs peak/operating season of your seasonal park comparisons. I'm not quite sure that they would surpass SeaWorld even by that metric. Obviously if you count the slow days of the year, that might change things, but they still have seasonal staff. Its hard for me to believe that SeaWorld cutting its operating days will improve its situation any... it will just further increase attendance declines.
And 2009 is hardly irrelevant... its not that long ago in the grand scheme of things, and in the case of Universal vs SeaWorld, we're literally comparing the same ownership group causing massive financial problems and associated attendance declines at the parks, just over different time periods. And from a name recognition stand point, I'm sure many, many more people know of SeaWorld then Knott's