Prob stay around the resort, snorkeling would be on the list.
I don't know about any resorts, because we always do cruises. But here's what I can tell you about the western Caribbean:
In general, no resort is going to have decent snorkeling at the resort beach. There are exceptions. If a resort doesn't have good snorkeling at their beach, they'll offer excursions to somewhere close that does.
The biggie: safety. The US state department has basically declared all of Mexico unsafe. Take that for what it is worth - Cozumel, Cancun, and Cabo San Lucas are all still popular vacation spots.
If that doesn't scare you away, I love Cozumel. I've done snorkeling, snooba, and scuba there, and it was very good. I've never felt unsafe (but I tend to mind my surroundings when out and about). I personally would not hesitate to spend a week there at a resort. They drive on the right side of the road. Makes renting mopeds and cruising around easy. Private beach spots range from family friendly to frat parties complete with topless women. Chankanaab park and Punta Sur park are beautiful.
If safety is a concern, have a look at Grand Cayman or Punta Cana.
I've been to Grand Cayman several times, and I always hit cemetery beach. Absolutely beautiful stretch of beach, and the best snorkeling I've ever done. There's another spot right in the middle of town where the cruise boats disembark that is supposed to be awesome snorkeling as well. Being a former British colony, they drive on the left, which makes renting mopeds and cruising, um, interesting.
I've not cared much for Jamaica, but I haven't seen much of it. On the other hand, I did go to Sunset Resort for a day, and their beach was one of the best touristy beaches I've ever hit, and the snorkeling right off the beach was pretty good as well. The resort itself looked a bit iffy - I'm not sure I'd want to spend a week there. Food looked ho hum, and the drinks were pretty shabby.
You might want to consider a cruise. We've done several in the western Caribbean, and we've loved them all. If you can live with a small amount of alcohol and pop while on the boat, cruises are relatively cheap.
If you have any questions, feel free to ask.
You also may want to have a look at a web site called "cruise critic". There is a very active forum there, and there are threads on all of the popular spots in the Caribbean.