http://www.dolphindiscovery.com/cozumel.htm
depends on what you want to do... the Dolphin Encounter is only available through the cruise line on Wednesdays... as all times are allocated to the cruises...
Here is a discussion that I had just yesterday with one of the people at Dolphin Discovery:
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info: You are now chatting with 'Marian'
Marian: Hi Dawn, how can I help you?
Dawn: The dolphin encounter is not available on Wednesdays in Cozumel?
Marian: No - the allocation is given over to the cruise ships. Are you coming on a cruise?
Dawn: Yes...
Disney Cruise
Marian: Have you checked availability with them?
Dawn: I will do that... just wanted to arrange our excursions on our own. One more question...
Marian: SURE
Dawn: Is it possible for my daughter to participate in the Swim... we will be there on Feb. 5th and her 8th birthday is Feb. 16th
Marian: Yes could participate as long as she had an adult participating with her.
Dawn: She is such a nature and animal lover... I just know she would love it!
Marian: She is too young to do it on her own.
Dawn: That's great... I thought she could not participate at all unless she was 8.
Marian: We don´t ask for proof of age, and if another adult is with her, she should be fine.
Dawn: Thanks! Have a great day!
Marian: You´re welcome - you too.