If I were you, I would book with an outside vendor. There are plenty in Cozumel, and most of them are very reputable shops and dive centers (if you're concerned about finding a good one, there are plenty of reviews online and different message boards that you can check out). If you book you're own excursion, and have a enough of you to have a private to semi-private charter, it would be better as you will have been certified for only a few days and, i'm assuming, is going to be your first dive post certification. This way, you can let your divemaster know that you are all newly certified, and he/she will take you to appropriate dive sites. And, you will also not be in such a large group.