I stayed in the Myan but wasn't impressed with the condition of the local reefs near by. If you stay anywhere near Playa de Carmen or Cancun you really need to take full day trips to see the good stuff, absolutely nothing off the resort shores. Also a hurricane was through there recently and when I was there in May you could see it certainly had a big effect on the reefs. Diving will be much better than snorkeling.
Cuba was good, I was at the same place as Colby. The snorkeling off the resort was good and the diving was also very good. Diving was a little sketchier than I like, equipment was fairly old and likely poorly maintained. On every dive someone had some kind of leak problem, I myself ran out once due to a leak from the tank valve. But I wasn't ever really that worried, but my gf was pretty freaked out the whole time. Actually most of Cuba's stuff was a little rough around the edges, I recall breaking through a net on a catamaran and being tossed in the open ocean, ahaha but if I could back I wouldn't change a thing. Good thing was we used the dives as certification dives and didn't have to do any skills
Not too many fish except for the lions, which were certainly the highlight of the trip, but lots of unusual corals if you looked close enough. I also have pictures posted
here.
I've also dove in Jamaica which had Padi certified shops and much better equipment. Diving was OK but I saw more in Cuba, but it also depends where you go and what you're looking for and hoping to see. They have a few nursery reefs around the island which are the best I've seen to date, great for snorkeling and quick ride on the boat from the resort. Snorkeling off the resort was good but it depends where you go, we went to the Gran Bahia Principe in Runaway Bay, it was brand new at the time so the reefs off shore where still in good shape.
We're now planning a trip to the Dreams La Romana Resort & Spa in the Dominican Republic. The snorkeling off the resort is suppose to be awesome and I've seen a few pictures that look promising. They are also suppose to have some of the best dive sites in a Caribbean close by. We chose this resort mainly for the snorkeling and diving opportunities but also because the price is reasonable.
Good luck with your decision, no matter what it'll be a good time.