No, you don't have to pay for your admission ahead of time; you can pay at the time of your visit, but depending on the day/time, you can save time by signing the waiver online. They can get pretty busy.
Tickets are not required ahead of time. When you go in, you will have to fill out the waiver at the side kiosks before waiting in line. I would suggest filing out the waiver online before going in, to save time. You can also find discounted tickets on Groupon.
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