Since the only criteria you listed in your post is snorkeling you might find the following to be helpful:

1) You’ll find excellent snorkeling on Bonaire but you might find the beaches there to be a bit of a disappointment.

2) The British Virgin Islands have many areas with very good snorkeling and some very nice beaches.

3) There’s some very good to excellent snorkeling in the Turks &Caicos as well as some of the most beautiful beaches in the world.

4) Many of the Bahamas put islands have very good snorkeling and wonderful beaches.

In order to give more specific recommendations it would help if you gave us some idea of when you are planning to travel, how long a trip are you planning, where you are traveling from, how much are you willing to travel to reach your intended destination - are you willing to take one or more connecting flights or ferry rides or make an overnight stopover to reach your destination, are you looking for only “walk-in” snorkeling or are you willing to take day-sails or organized boat trips to snorkel off-shore reefs and, most importantly, what type of budget do you have for air fare, accommodations, food and activities- please give specific dollar ranges and not some nebulous terms like “affordable”, “not too expensive” or “won’t break the bank” as without a point of reference these terms are meaningless.