I emailed them and they confirmed that there is no separate entrance and they do not set aside entrance for passholders. They do, however, allow pre-ticketholders and passholders in at 7:45 am.
See message below for those interested...
Dear Guest:
Thank you for your interest in Aquatica, SeaWorld's water park.
We do not have a separate entrance for passholders and we have reached capacity often, but close the gates to all guests. The demand for Aquatica visitation is too great to try to segregate visitors. We do open at 7:45am for pre-ticketed guests and passholders and are open until 10pm.
Thank you again for your letter. If you have additional, questions please do not hesitate to contact us. We look forward to entertaining you and your family at Aquatica.
Sincerely,
Aquatica Guest Relations