Southwest's frequent flyer program is called Rapid Rewards. You can sign up online. Once you finish all the questions, a "card" will appear on the screen. You can print it off and bring it with you to show at the Sea World ticket booth (in Orlando) for 20% off. I don't have my receipt here, but I believe it was about $43 for adults and maybe $33 for kids. It worked out to 20% off the full admission price per ticket. Yes, it's a GREAT price. It's good for up to 4 people per southwest card.
Yes, you can take advantage of the 2nd day free. In fact, SeaWorld employees tell you about this at the gate. You just go to the booth inside the gate (to the left), show your ticket, they take a picture of you and give you another ticket that says 2nd day free within 7 days (with your picture on it) - and it prints off the good through xx date. We signed up for a 2nd day free as soon as we entered. Unfortunately, we never made it back.
I'm not sure if Southwest will send you another card. I have an old hard Southwest card. I signed up online (for my son) about a month ago in case we needed more than 4 tickets. I haven't received a card in the mail yet.