We love arriving at Aquatica the afternoon. Many people seem to leave starting at about 2pm. By 4pm, it really starts to empty out. We stay in the water until the park actually closes. Most people are done with the showers by 15 minutes after closing so we seldom have to wait for them.
The only proplem with afternoon visits is that storms do tend to pop up with the heating of the day, so the weather could affect your plans.
The crowds at Seaworld are hard to predict. Since it is popular with families with kids, it may not be so busy at opening. Make sure to see the Penguins before the line gets too long.
Last time we were at Aquatica, it was filled with Brits. It can be busy with foreigners on school break when kids in the US are still in school. I'm not sure when their school breaks are.