I've stayed at Casa Mexicana. You're right about the social vibe and the nice proximity to the restaurants/square. The breakfast buffet is really great.
One consideration about Casa Mexicana is that it didn't seem set up for divers. It's more of a business class hotel. I'd also highly suggest getting an ocean view room for wetsuit drying, etc. Finally, if you're diving with an op that does valet gear stuff and has boats that leave from the downtown dock, you're in good shape, otherwise it's not very convenient. It's about a block from the hotel to the dock, the hotel is one story above the street, and you either have to take stairs or ride an elevator to your room. They have/had an escalator to get you up the first story to their lobby. Oh, and their "pool" is only about 3 feet deep, if that, and appeared to me to basically be for show. It *is* a nice place though. And super convenient for eating at the different restaurants.
Others will can correct me, but if I was staying downtown, I'd probably do Vista del Mar. It's probably less expensive and I think they have an agreement with Casa Mexicana regarding breakfast.
One other possibility is the Hotel Barracuda. It may have already been mentioned. As I understand, they have a busy bar area. (social consideration) It's a very easy walk "to town", really it's "in town", and either Mega or Chedraui is very close.
-Blair