I swam with dolphins when we were in mexico, we stayed in cancun and did the trainer for a day at Parque Nizuc, it does have a swim park with all the flumes next to it, but the dolphins are actullaly out at sea. I felt we spent alot of time with them, we also did a wee bit on anatomy and feeding etc which was intereseting. We got 3 photos - on with two dolphins kissing you! and if you wanted a video of the day you had to pay more. There was a swimming with dolphins beside on of the shopping centers, but i felt parque nizuc looked to be the best option.
Mexico itself was an amazing holiday and i would highly recommend it - we stayed for two weeks, and also visited the Chichen Itza which was a fantastic day trip; we took a boat trip over to isla mujeres aswell.
Hope that is of some help, and good luck with finding the perfect dolphin experience - you won't regret it - it is the most amazing feeling in the world! xx:green: