They will have more than enough snorkeling equipment for rent when you want it on Castaway Cay and they will just put it on your Key to the World Card. Personally, I would bring my own as it will be cheaper than renting. My recommendation is that if you cannot bring some from home due to space or baggage fees, is to go to Walmart or like place and buy a cheap set before boarding if you can. It will likely be cheaper than renting. You can always leave it behind if returning space is an issue! They will provide you for free to use a yellowish/greenish. BC (vest) that you must wear when snorkeling.
I am an active diver and bring my own mask snorkel and fins to the island, but I use dive charters gear when on dives as I do not want to deal with carrying it in most ports. On castaway, it is an easy carry and drop off at a spot you want to sit at.