If you have the opportunity, you should definitely swim with the dolphins. I know that Ixcaret and Isla Mujeres both have programs available. Since the numbers are limited of how many can swim each day, it's probably easier on Isla Mujeres. They have more dolphins available, and reservations can be made in advance. They are beautiful, friendly animals and you will never forget the experience.
If you ever have the chance to go and swim with dolphins, just do it. We did and what an experience it was!!!
We visited the Interactive Aquarium halfway the Cancun hotelzone, because we were told that this is a place where they treat the dolphins very well. (At some other places, dolphins are rather stressed because of the large amounts of tourist they have to "entertain")
We spent half an hour in the water, with sometimes one, sometimes two dolphins shared among 6 persons. Highlights were the dolphin kiss (beso), see my profile pic. and the footpush, where two dolphins push you at your feet across the basin. What a feeling this is!!!
Bring an underwatercamera for some snapshots while you are in the water. And we even filmed eachother during the footpush. The pic is a downloaded still from my filmcamera, so that's why it is not so sharp.
Apart from the dolphins you have also access to the rest of the aquarium, where you can stroke rays, starfish, sea cucumbers, etc. as well.
Remember, it is quite expensive, but a once in a lifetime experience.
You must go and visit that place,
you can swim with dolphins, or have educational interactive program, as I took.
You can learn a lot about that beautiful animals.