I have been a PADI rescue diver for four years and have been diving in Cozumel twice. Although the reef is not what it used to be before Hurricane Wilma, it is still a BEAUTIFUL place to dive and the people there are FANTASTIC!
I have been certified for 18 years and agree that it is the most fun you can have. I have dove in the keys, hawaii and great lakes. If I was you I would go get your certification and then go on your trip. Snorkeling is fun but not as fun as diving.