B_D's suggestion is probably best because it is closest. Back in the day, Dive Paradise had a beach location at Hotel Barracuda. If they still do, then this is a much better shore dive than in front of El Cantil/Palace/Hotel Cozumel. Actually it can be a great dive, although I haven't done it in about 8-10 years. Just walk out El Cantil and turn left. It's a short walk.
We just got back from staying at El Cantil. Always wanted to stay there, but we never really needed a bigger unit. They now have "Penthouse" units that are 1-bedroom where Prima's restaurant was. The rooms were just OK. I would never stay there again. The location is fantastic though, we walked almost everywhere.