Got diving in Cozumel. I agree you wont get much with snorkling but i have disagree that you wont see anything diving . I saw a variety of brain corals and mostly sps. Also, a variety of tangs and i did see a couple of Achilles tangs. Remember to go when the waves aren't that big ( sometimes it gets stormy for no reason). Lastly most of those hotels are all inclusive so if your diving/snorkling trip isn't successful hit the all inclusive bar and it usually improves your mood.