We visited Discovery Cove in July 2001. Although expensive, it was one of the best days we've had on vacation. The kids, DS was 10 and DD was 12, absolutely loved the snorkeling, and swimming with the tropical fish and rays. The park is completely gorgeous and once inside you'll forget you're not at a real beach! The aviary is amazing too! We arrived at the park at the opening and stayed until just before closing and were never bored. (I even got to lay on a beach chair in the sand, overlooking the snorkel pool, and get a couple hours of sun while DH, DD and DS swam without me. Wahoo!)
Lunch was included in the cost of admission. While we fully expected basic fast food type fare, DH and I were pleasantly surprised with "grown-up" food! There were several choices of hot or cold lunches as well as dessert and beverages, not just soda either! The food was truly delicious and I would have expected to pay $40 to $50 dollars just for the bountiful lunch we had in the park.
Obviously, the reason you go is for the dolphin swim. I must tell you that it was one of the best times I've ever had and the family agrees. Sharing the experience with my DH, DS and DD was remarkable and I find myself a little moved remembering it. You must spring for the pictures of each of you with the dolphin as well. It is a keepsake you'll treasure.
Bottom line? If you can possibly fit it into you budget, do it.