Yes, every time we go we do the add a second day for $10.
I don't know what happens. I think it's the nature of Sea World being lots of shows, and it's just hard to fit them all in. We always think one day will be enough, but then end up needing more time to finish everything up. There always seems to be one or two shows we just can't fit in. Then we think another half a day should do it, only to decide once we go back that we just have to do everything we missed, plus all our favorites again, and so end up spending a whole other day there. Two days total.
We are probably the only ones who do this though. I really love the ocean and aquariums and the whole laid back feel of Sea World. I'm sure I could plan better, and we could fit it all in in one day if we just rushed a little more. I've just never seen the point since it seems like a no brainer to me to get two days for basically the price of one, so why not take my time and enjoy it. Plan nice relaxed meals, have a snack from the bakery or ice cream shop, see the shows again, window shop, re-ride Manta, let kids play in climbing structure, enjoy the new sculpture garden, etc., in general just take our time doing whatever we feel like.