I have been to Cozumel three times, once as a port stop while on a cruise, and twice by air. The first time I saw Cozumel was while on the cruise and I loved it some much, I have been back twice more to stay longer. On the cruise, we chose Paradise Beach as a shore excursion. I liked it, but had I known that there was better beach and water at San Francisco Beach and Chakanaab (sp?), we would have chosen something else. Paradise Beach is laid back and quieter than San Francisco, mostly because it attracts the cruise ship visitors more than the land visitors. If you want a wilder, more party atmosphere, then San Francisco Beach is where you need to go. They have more water sports than Paradise Beach. If you want excellent snorkeling, then you should go to Chakanaab. (not sure of the spelling). The fish are usually waiting for you to come into the water. The little ones gather by the stairs, hoping you have some food. (However, I wouldn't recommend feeding them. There are a few baracudas that hang around there.)
Hope you have a grand time. I get to go back on my 9/27 Fascination cruise and I can hardly wait.