SeaWorld honors the Rapid Rewards Membership card by Southwest Airlines.
This is just a membership card (free) and by showing it at the admission ticket office, they give you 20% off on tickets to Sea World.
I have used it twice, recently June 2009. This last time I did not have my RR card with me, but showed them a couple of emails (on my IPhone) from SW in reference to my RR account and they accepted that.
If you have never signed up with Southwest, you can sign up on online and print a temporary card immediately. It is free and you can get the card even if you don't have a flight planned.
You only need one RR card in order to purchase several tickets.