The SeaWorld website now lists 2 different single day tickets.
#1 SeaWorld Orlando Single Day Ticket with 2nd Day Free -- With this ticket you can visit once and come back for another day within 7 days of your first visit, all for the price of single day admission. (59.75 adult, 48.00 child)
#2 SeaWorld Single Day 10% Discount with 7 Day Advance Purchase -- Save 10% off single day admission to SeaWorld Orlando with this Print@Home ticket. Minimum 7 day advance purchase required.
Note that ticket #2 (the discounted ticket) doesn't say anything about a 2nd day free. If you look down this thread, DisneyFan#1 reported that when they were at SeaWorld recently only full price tickets were eligible for the 2nd day free within 7 days. And metta4 said they just called today and SeaWorld confirmed that only full price tickets were eligible for the 2nd day free within 7 days. Previously, when purchasing tickets online, there was never anything stated about a 2nd day free within 7 days. From the way it appears, I'd say it is definitely true that the only way you'll get the 2nd day free within 7 days on a single day ticket is if that ticket is full price.