I suggest you contact Eagle Ray Divers in Cozumel (I think its
www.eagleraydive.com, if not do a yahoo search on Eagle Ray Dive shop in Cozumel). Whereas we have never taken a snorkling trip with Eagle Ray, my 17 year old son and I took a dive trip with them this past summer. Cozumel is GREAT for seeing lots of fish and coral. Others on this and other boards have commented on their positive experience with Eagle Ray on snorkling trips. Chellie at Eagle Ray is very professional and will provide a top-notch experience for your daughter.
You say this is your daughter's first time snorkling? Does she have her own equipment? Has she tried snorkling somewhere else before (i.e., a pool)? The reason I ask, is that my DW tried to snorkle on our honeymoon, 25 years ago. However, it really bothered her to breathe through her mouth, which is required to snorkle. Whereas my daughter, son and myself snorkle all over the Caribbean, my wife is not comfortable with it. Thus, I would highly recommend that she buy her own equipment and try it out in a pool to see if she's comfortable with it and wants to do it on her cruise, before you put down money for a snorkling trip and THEN she finds out she doesn't like to snorkle. (maybe instead of just snorkling, your daughter will want to get her scuba certification, Cozumel is known the world over for its great coral reefs and scuba diving. My son and I took the lessons the summer before last and we had a great time toghether and now its something we can share).
Steve