It's definitely worth if if you take advantage of Sea World. When we did Discovery Cove in 2002, we actually went to Sea World twice, because admission was included for 7 days. (this was a 10 day trip, with the other 7 days being spent at WDW!). If you really want to interact with dolphins,then yes, it's worth the price even without the inclusion of Sea World. My family couldn't get enough of the snorkeling, the bird sanctuary, just swimming, and the quality of the food at lunch was very, very good. We had great dolphin trainers, who really made a big deal of my kids (we were lucky as the other people in our swim group were 2 adults.) We did Discovery Cove after 4 LONG days at WDW, so this was a very welcome relaxing day. We are thinking of going back, and scheduling their Trainer for a Day program.
If you do decide to go, get there early, as dolphin swim times are assigned (or at least they were in 2002) on a first come, first serve basis. We were there about 30 minutes after opening, and our swim time was at 3:20 pm! Also, try and schedule this for the middle of your trip if you can. They do cancel swim times in the event of lightning storms. If you don't get in that day, then you can come back the next day. We heard of people who left this for the last day of their trip, and had to receive credits for a future trip. Best to plan for all events! Best thing to do is get to Discovery Cove EARLY.