Hotel Cozumel or Villablanca cater to divers and often have discounted dive packages. Nice enough rooms and pool, etc. that's not too far from town. Intro shore dives from the dock. There should be a mix of dive experience people on the boat to team up with. Last February I wore a 2mm hooded vest under a 3mm full suit. 79 degrees for 1 hour, I needed it. My wife wore 3.5mm with an added hoodie.
Good advise, indeed! In my youth I wore a 3mm suit when diving Cozumel in Jan. I have now "graduated" to a 5mm suit with a 3mm hood.
The thing to remember about Cozumel is that nearly all the diving is live boat drift diving. That is the boat drops you off and you drift with the current. The boat follows the bubbles and is, in theory, at the end of the dive in the right place to pick you up. Many of the dive ops use the "loose buddy" system in which you go up when your air says to. Thus, folks with good bottom times are not held back by those with less extensive bottom times. That said, the dive ops there are very used to novice divers and will keep a close watch on you. Nevertheless, your safety is always your own responsibility.
joewr