Have you thought about Hawaii? If this is the first time you're going on a trip it might be a little more stress-free. Nonstop flights, no language barriers, etc. And tropical
When I was in Mexico the resort I went to had a decent kids place (I went before I had kids though), so some all inclusives might be worth checking into, if going to the Carribbean is still your first choice.
Snorkelling ... well .. I'm not sure what to suggest here. You won't really be snorkelling a lot if you have small kids with you and you're just going by yourself (ie., just your family). Snorkelling alone isn't responsible, my suggestion, don't do it - you need a dive buddy. Even in a crowded area like a beach, if nobody is actively actually watching out for you (and you back at them in return) then it's possible to slip into trouble without anyone noticing. Especially things like riptides, you don't know what hazards exist in an area you haven't swam before.
Although another option is booking yourself into a dive group done through a local dive shop.. sort of takes care of that issue.
When I went to Maui with Nathan in 2006, he was 1, but we were there as a family reunion so I had lots of family onhand to help watch over him when we wanted to do something like snorkel. Having said that, we still had to miss out on a lot of things that the famdamily could do just on account that you can't haul around a 1-year old or 2-year at a pace that you can do with older kids (or no kids). So it definitely takes some planning.
Personally, I suggest seeing if you can talk the grandparents, or an aunt or uncle, or friends who have kids too in similar age, in going with you at the same time. For one, it's funner with a small group, and for two, you can at least trade off the parenting duties so that you can do things like snorkel and whatnot. Just make sure that the parenting duties are equally shared because it's no fun for the one parent who is constantly "left behind" to tend to the wee ones.