First rule about snorkeling with a
DCL excursion - everyone is required to wear a Buoyancy Compensator (BC). It's like a real light life jacket. You can inflate it with as much or as little air as you want. Just a little really keeps you afloat, so you don't have to worry about finding someplace to stand and rest, you can float and rest!
We snorkeled in Key West, or actually about 8 miles off of Key West. It was a DCL excursion. We took a fury catamaran out to the reef. The water was pretty choppy. My son got several snorkels full of water before getting the hang of it (the trick here is to look straight down or a little up - but NOT back towards your feet - the snorkel goes underwater.) The reef was great. Lots of sea critters to look at. No place to stand here the water was 15 to 20 feet deep. Trivia point: it's the 3 largest barrier reef in the world, the first is in Australia (duh!) and the second is in Belieze.
We also snorkeled in Grand Cayman. We did the Shipwreck and Reef snorkel. We snorkeled right in front of where the Magic was anchored. It is a great place for beginners. The water was very calm. We stopped at the shipwreck first. Here the crew tossed in some fish food and is was like swimming in a very crowded aquarium. Way cool. Then we moved a quarter of a mile or so over to a reef and snorkeled there. No fish food here, so we were able to see the fish around the reef. The reef and sea critters were awesome. They have fish called tarpins that look like sharks that hang out at the bottom of the reefs during the day. When you first see them it's kinda scarry. Overall - GREAT EXCURSION!!
Doing Captian Marvin's or Bryons is a good way to get to do some snorkeling and see the sting rays too. Another option is Nativeway - they have had some great reviews on this board.
During one of the days, the crew of the Magic give beginning snorkeling lessons in the Goofy pool - see the navigator for times.
If you are going to WDW before your DCL cruise, you may want to spend a day at Typhoon Lagoon. They have a snorkel attraction there (you snorkel with the sharks!!) That would also give you some basics.
Generally speaking our excursions consist of snorkeling, snorkeling, snorkeling, snorkeling, and more snorkeling! I don't understand the going to the caribbean and doing the shopping, shopping, shopping thing.
Good luck and enjoy learning to snorkel.