The port of Cozumel is really nothing more than thousands of trashy souvenir stores and bars. Great for the first time you visit, boring after that. We elected to visit the east side of the island, 30 miles of absolutely uninhabited coastline. (It is ALL a national park) What has been permitted is a FEW small, simple beach bars. The best way to see them is the Bar Hop. Fun, laid back, unhurried, and NOT the drunken mob I was fearing. A great way to spend the day.
If you've never been there Key West is fun. If you HAVE been there, there is not much to do. This time, we just wandered the town (Absolutely safe) and watched the parade of humanity wander by. Of note, we did stop by a quirky and wonderful little restaurant named "Blue Heaven". It's a GREAT place to spend a few hours away from the hustle & bustle of the port.