I will freely admit to being more than a little biased, mainly because I've been fortunate enough to experience some of the dolphin swim experiences outside the USA (southeastern Mexico). Now, with that said...
It really does depend on what you're after. Discovery Cove has one of the best free-flight aviaries I've seen at ANY park in 25+ years of traveling. If you like birds (and small deer), definitely go for the aviary. You'd also (probably) like it if you like coral reefs and their various inhabitants. Swimming through a pool while you're literally surrounded by various types of tropical fish and rays is pretty neat.
Their dolphin "swim," well, that's a bit different. While I will grant that it may have improved (Essence, I hope so!) since the poor experience that myself, my wife, and some friends had in 2002, I have no way to know for certain.
I will say that, during our visit, the so-called "swim" seemed to be much more about contrived photo-ops, selling overpriced baubles (post-swim "debriefing"), and hyping how great the Almighty Sea World is than it was about giving people any actual 'swim' time or an honest experience with the animals. If you do go, I would make it a point to observe -- very carefully! -- one or more of the actual dolphin sessions before signing on the dotted line.
My take on it, assuming nothing has changed: Go for the aviary, river swimming, and beaches. Don't bother spending the extra $$ for the dolphin stuff. If you still want to swim with dolphins, you'd do far better going further south into the Keys (more specifically, to the Dolphin Research Center, Theater of the Sea, or Dolphins Plus).
Happy travels.