I think a good first source of information would be to visit our sister site, PortReviews.com. There you will find answers to many of your port questions as well as a great number of suggested activities. Just go to: www.caribbeanportreviews.com
and then click on the links on the left for the desired destinations. After you have read that info, if you have any other questions or would like additional feedback, this is the place to get those questions answered.
By the way, if you have not visited Cozumel before, there is so much to see and do right there, I would save a Cancun excursion for a return trip.