We did Discovery Cove a few years ago, wanting to swim with the dolphins.
The actual one-on-one swimming experience with yourself and a dolphin lasts about 15 seconds. You swim out a few yards, they call a dolphin, grab hold of a fin, it pulls you in a few yards, thats it.
You can get to hold their snout an kiss them and have your picture taken whilst doing it (costing $$ for photo photocd), but you won't get much more than that.
Discovery Cove with everything else there, is a good visit. But if you go there with the intent of actually spending some time with these creatures, its a bust.