You can do what they call a "dive experience" where they give you some quick lessons, put you in the gear, and you dive with a dive master by your side the whole time. There are some limits of how deep you can go, and how long you can stay down, but see if you can get a "dive experience" rather than a snuba. All of the tourist places you will see in cozumel will be good to visit. Tourism is a huge part of their economy, so they are good at making you feel welcome and happy. The little shop vendors will do all kind of things to get you to come into their store.
If you only have a day in cozumel see if you can set up a dive apointment with these guys
http://www.liquidbluedivers.com Roberto is the owner and see if he can take you diving that day. It may even work out where they can take you on a dive, then leave you on a cool beach for an hour or so while others do a second dive. Not sure if he can do that or not, but it maybe worth asking.