I've actually done it twice. My first dolphin swim was in Key Largo. It was at a Dolphin Research Center. The experience was definitely one to remember, and probably the best ones I've ever had! It wasn't as extensive as a 2-day experience (although now that I think about it, there might have been an option for that.

) but it was decently priced (I think about 120.00 - but that was 10 years ago!) We were in the water with the dolphins for about 2 hours, and got plenty of one-on-one time. There were only 4 in our group. They teach you commands and then let you in the water with them, almost freely, (under the trainers watchful eyes, of course)and you can mix up the commands however you like, and interact with them at your leisure. I had 2 dolphins with me, personally, during my experience. It is held in a cove that leads out to the ocean, and there are places for others who aren't doing the experience to sit and watch, take pictures, etc.
We tried Discovery Cove a few years ago, when I wanted DH to try the experience. While he was impressed being a first-timer, (I don't think anyone can have a "bad" time swimming with the dolphins!) I was less than impressed. The facility is beautiful, well kept, and there is lots to do. The crowds are limited, so that's great. They have a place you can swim with the sitngrays and other tropical fish, as well as a lazy river. You are allowed to spend your whole day there, and it is VERY relaxing, but the dolphin experience itself was very structured, and the time you got with the dolphins IMO was more so they could get a posed picture of you to take home rather than enjoying the experience itself. The dolphin experience lasts about 45 minutes, and you share that time with one dolphin and about 6-8 people. The put you in "pods" around a pool, and there are other pods of people with their own dolphin in the same pool. The trainer does most of the tricks, and we got to swim once around a small area with them. All in all, a good first time experience for DH, but it didn't hold a candle to my previous encounter in Key Largo.
Here is the website of the place we went to:
http://www.dolphinsplus.com/
Hope this helps, if you have any questions, feel free to PM me - I love swimming with the dolphins!