The second day free offer isn't listed on the SeaWorld website anywhere. It is really just an in-park promotion. As you leave the park, but before you exit, there is a building on the right with a sign that says something to the effect of "Get your Second Day Free Here" and has ticket windows just like when you enter. You can turn in your single-day ticket, get your picture taken and they print you a new ticket with your picture on it that can be used within seven days. I believe you can also apply the cost of your single day ticket toward an annual pass, or a multi-park pass with Busch Gardens in Tampa, at this location.
I guess they could discontinue the second day free at any time, but they have made fairly permanent accomodations for it at the park exit.