To be honest, snorkeling in Cozumel is not that great. There are some places where it's okay, but you have to understand that fish like coral reef and rocks - it provides them food and shelter. The coral reef in Cozumel doesn't start until around 45' in depth, which is a bit deep for snorkelers to enjoy the fish.
But because of the amazing coral reef system in Cozumel, it's one of the best places in the world for scuba diving! I've dove there many time and it is absolutely awesome!!
Chankanaab does provide some decent snorkeling, but still not as great as other places such as Roatan (which is awesome), Belize, or Grand Cayman, just to mention a few.
Hopefully, your itinerary will take you to some island that provide better snorkeling.