we went to florida last year mid august august and our first full day was a monday, and we decided to go to seworld. its was empty!!! the stadiums were still mostly full, but all the rides like wild artic, kraken and journey to altantic were quiet if you do them in the morning as they are quiter as your able to walk straight on.
I would recommend any time early in the week. SeaWorld is flooded on the weekends with Florida residents like myself who hold annual passes. The park is getting more crowded these days. 5 or 6 years ago it was easy to get into and around in, but lately it stays pretty full. I guess the marketing guys are doing thier jobs.