Reefd8vr,
I don't remember noting Casa Mexicana specifically when I was there, but based on the map it looks like you're practically in town. If you're looking for nightlife a la E!'s "Wild On", you probably want Senor Frog's, which should be very close to where you're staying - it's on the Oceanfront main drag - you really can't miss it. Cozumel isn't like Cancun (or at least what I've heard of Cancun), so if you're expecting Mardi Gras-like celebrations, you'll probably be disappointed.
There are some public beaches a little north of the main square, but I'm not sure if they're worth diving. Near town, it's a more high traffic area - I don't think it's really a good idea to try to dive there. Your best bet might be to take a taxi to Chancanaab park (there's an entrance fee). When we were there, we stayed at Hotel Cozumel, about a mile south of the main square. We had to walk a bit to get into town, but the hotel had a private beach frontage that you could snorkel or scuba dive. Not as nice as the boat dives, but not bad either (better than Wisconsin's vis). You can get a taxi to this area, but I don't think there's a good way to do the entry from the public frontage.
