You can use your own equipment. We bring our own - We'd more than pay for it in rental cost otherwise - our last cruise (Snorlkeling in KeyWest, Grand Cayman, Cozumel, and Castaway Cay). It doesn't take up that much room if you pack right.
We brought our own. I bought the kids snorkeling equipment that included fins - they were really cheap at BJs (I think Costco has them too). For my husband and myself I bought the cheap Walmart ones without the fins. We just rented fins when we wanted to snorkel. Yes, you can snorkle at Castaway Cay with your own equipment. I prefer using our own - I would never rent again! It saved us a lot of money and I know that I sterilized it before we used it!
Yes, you can bring your own equipment and use it at Castaway Cay. You can use their vests (mandatory) for no charge. After the Norwalk scare I was hesitant about using equipment that others had used even though it was supposedly disinfected. There are five of us so we use a rolling duffle bag for all our equipment. HTH
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