There is a sign before the toll plaza saying the parking lots open at 9:15AM, so I assume the front gates would open at 9:30, 9:45, or 10AM depending on the expected attendance for that day. Busch Gardens Williamsburg doesn't always open up early. Sometimes the turnstiles don't open until 10AM, but that typically happens when they do not expect a large crowd. I assume Water Country USA works the same way.