Hi again, Melissa! This fall sounds like the perfect time to escape to the Caribbean for some warm weather and fun adventures aboard Disney Cruise Line! You'll love all of the ports in the Western Caribbean itinerary, and the amazing Port Adventures that they offer.
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Port Adventures that include snorkeling do have a variety of sizes of life vests and snorkeling gear, including masks and fins, so no need to bring your own. However, many families do like to bring their own simple gear just because their children use that at home, they prefer the style or fit or a number of other reasons. So if you think your kids would prefer to practice at a pool at home and bring their own special gear that you know fits exactly the way they like, go for it! Since my family only snorkels every now and then on a vacation, we just use what the Port Adventure provides and it's always worked out well, plus it's one less thing to pack in my already overflowing suitcase!
Melissa, you still have the whole summer to plan, so feel free to stop back here with any other questions as you continue getting excited for your Fantasy cruise vacation!